fix(lanes,git): harden the lane git surface and fix client races in the Changes panel

Server (git-diff.js, git-ops.js, worktree.js, routes/lanes.js):
- Every lane git call runs with GIT_LITERAL_PATHSPECS=1 and -z porcelain, so a
  file name is a file name, never a glob, and odd names parse correctly.
- A lane that is a subdirectory of its repo sees and acts on ONLY its own
  paths (reads, stage/unstage/discard/commit, and gitFacts.files).
- Any file named settings.local.json (the lane's router API key) is refused as
  403 ESECRET on disk (symlinks resolved) and at every committed rev.
- commit refuses with 400 ESTAGEDOUTSIDE while anything outside the lane is
  staged (checked with --no-renames, so a move into the lane is caught).
- push resolves its remote (upstream → only remote → origin), else 400
  ENOREMOTE; unstaging a rename unstages both halves.
- Git writes serialize on the repo's git dir, not the lane id, so two lanes on
  one index cannot collide on index.lock.
- git refusing an action (a numeric exit status) answers 409 EGIT with git's
  stderr instead of a 500 carrying a numeric code; one shared parseNumstat.

Client:
- Changes panel: no stale answer from a previous lane or an older quick-open
  query repaints the panel; discard disarms on lane switch; Enter on a file
  path opens it instead of folding; bounded tree expand-all.
- Large diffs (> 500 rows) render only the visible rows, with stable width
  and in-sync split columns; mirrored split scroll is clamped.
- Git bar: no "in sync" without counts, a clear no-remote message, branch list
  re-read after a switch, and no previous-lane facts after a lane switch.
- Tooltips no longer flicker or close on unrelated scrolls.

client/src/components/Tooltip.tsx

@@ -9,10 +9,13 @@
  * touching every one of those call sites to get a designed bubble, and the
  * next `title=` someone writes would be back to the browser's grey box.
  * {@link GlobalTooltip} instead listens once at the document: on hover or
- * keyboard focus it takes the element's `title`, suppresses the native tooltip
+ * keyboard focus (`:focus-visible`, not the focus a click gives) it takes the element's `title`, suppresses the native tooltip
  * by removing the attribute for as long as the bubble is up, and restores it
  * on the way out. Nothing else in the app has to know it exists, and a control
  * keeps its native tooltip (and its accessible name) when JS has not run.
+ * While the title is parked, a control with no other name (icon-only) carries
+ * it as a temporary `aria-label`, so focusing it never leaves it nameless; the
+ * bubble goes away with its control if that is removed from the page.
  *
  * ## The look
  * Anchored to the CONTROL, not the cursor — a bubble that chases the mouse is
@@ -39,8 +42,10 @@ const SHOW_DELAY_MS = 350;
 /** Gap between the control and the bubble, and the minimum viewport margin. */
 const GAP = 8;
 
-/** Opt an element (or a subtree) out: `data-tip-off` keeps the native title. */
-const SELECTOR = "[title]:not([data-tip-off])";
+/** Opt an element (or a subtree) out: `data-tip-off` keeps the native title.
+ *  `[data-tip]` matches the control whose title is parked, so moving between
+ *  its own children still resolves to it instead of re-opening the bubble. */
+const SELECTOR = "[title]:not([data-tip-off]), [data-tip]";
 
 type Anchored = { text: string; rect: DOMRect };
 type Placed = { left: number; top: number; arrow: number; above: boolean };
@@ -58,16 +63,29 @@ export function GlobalTooltip() {
   // that unmounts it, Escape).
   const parked = useRef<HTMLElement | null>(null);
   const timer = useRef<number | undefined>(undefined);
+  // The element whose delay is running, so re-entering it does not restart it.
+  const pending = useRef<HTMLElement | null>(null);
 
   const restore = useCallback(() => {
     window.clearTimeout(timer.current);
+    pending.current = null;
     const el = parked.current;
     if (el) {
+      // Drop the instance shim `open` installed; the prototype method is back.
+      delete (el as { removeAttribute?: unknown }).removeAttribute;
       const text = el.dataset.tip;
       if (text !== undefined) {
-        el.setAttribute("title", text);
+        // React re-set `title` while it was parked (a poll flipped the hint):
+        // that value is current, the parked copy is stale. React never
+        // re-applies an unchanged prop, so writing the old one back would win
+        // for good.
+        if (!el.hasAttribute("title")) el.setAttribute("title", text);
         delete el.dataset.tip;
       }
+      if (el.dataset.tipAria !== undefined) {
+        el.removeAttribute("aria-label");
+        delete el.dataset.tipAria;
+      }
       parked.current = null;
     }
     setTip(null);
@@ -79,7 +97,9 @@ export function GlobalTooltip() {
       const text = (el.getAttribute("title") || "").trim();
       if (!text) return;
       restore();
+      pending.current = el;
       timer.current = window.setTimeout(() => {
+        pending.current = null;
         // Re-read at fire time: the control may have been re-rendered, removed
         // or had its title changed while the delay was running.
         if (!el.isConnected) return;
@@ -87,6 +107,29 @@ export function GlobalTooltip() {
         if (!current) return;
         el.dataset.tip = current;
         el.removeAttribute("title");
+        // React removing `title` while it is parked calls removeAttribute on an
+        // attribute that is already gone: no MutationRecord, nothing to see —
+        // and `restore` would write the stale text back for good. So, on THIS
+        // element and only while parked, that call is taken as React's intent:
+        // the parked copy is dropped and the bubble hides. (React DOM calls
+        // `node.removeAttribute`, the instance method; `restore` deletes the
+        // shim.) No global prototype is touched.
+        el.removeAttribute = (name: string) => {
+          if (name.toLowerCase() === "title" && el.dataset.tip !== undefined) {
+            delete el.dataset.tip;
+            restore();
+          }
+          Element.prototype.removeAttribute.call(el, name);
+        };
+        // The title was an icon-only control's ONLY name; keep it named.
+        if (
+          !el.hasAttribute("aria-label") &&
+          !el.hasAttribute("aria-labelledby") &&
+          !el.textContent?.trim()
+        ) {
+          el.setAttribute("aria-label", current);
+          el.dataset.tipAria = "";
+        }
         parked.current = el;
         setTip({ text: current, rect: el.getBoundingClientRect() });
       }, delay);
@@ -101,7 +144,10 @@ export function GlobalTooltip() {
     };
     const onOver = (e: PointerEvent) => {
       const el = target(e);
-      if (el) open(el, SHOW_DELAY_MS);
+      // Already showing or already counting down for this control: a pointer
+      // crossing its children must neither hide nor restart it.
+      if (!el || el === parked.current || el === pending.current) return;
+      open(el, SHOW_DELAY_MS);
     };
     const onOut = (e: PointerEvent) => {
       // Moving between children of the same control must not flicker.
@@ -112,9 +158,14 @@ export function GlobalTooltip() {
     };
     // Keyboard users get the same tooltip, immediately — a focus ring that
     // reveals nothing is the whole reason icon-only buttons are hard to learn.
+    // Only KEYBOARD focus, though: a click focuses too, and without the
+    // `:focus-visible` check it reopened the bubble the pointerdown just
+    // closed, with no delay. Hover keeps its own path.
     const onFocus = (e: FocusEvent) => {
       const el = target(e);
-      if (el) open(el, 0);
+      if (!el || el === parked.current) return;
+      if (!(e.target instanceof Element) || !e.target.matches(":focus-visible")) return;
+      open(el, 0);
     };
     const onKey = (e: KeyboardEvent) => {
       if (e.key === "Escape") restore();
@@ -126,9 +177,17 @@ export function GlobalTooltip() {
     document.addEventListener("focusin", onFocus);
     document.addEventListener("focusout", restore);
     document.addEventListener("keydown", onKey);
-    // A bubble anchored to a rect is wrong the moment anything moves.
-    window.addEventListener("scroll", restore, true);
+    // A bubble anchored to a rect is wrong the moment its anchor moves — but
+    // only a scroll of the document or of a box containing the anchor moves
+    // it. A log tailing in another panel must not close the bubble.
+    const onScroll = (e: Event) => {
+      const anchor = parked.current || pending.current;
+      const t = e.target;
+      if (!anchor || !(t instanceof Node) || t === document || t.contains(anchor)) restore();
+    };
+    window.addEventListener("scroll", onScroll, true);
     window.addEventListener("resize", restore);
+    window.addEventListener("blur", restore);
     return () => {
       document.removeEventListener("pointerover", onOver);
       document.removeEventListener("pointerout", onOut);
@@ -136,12 +195,33 @@ export function GlobalTooltip() {
       document.removeEventListener("focusin", onFocus);
       document.removeEventListener("focusout", restore);
       document.removeEventListener("keydown", onKey);
-      window.removeEventListener("scroll", restore, true);
+      window.removeEventListener("scroll", onScroll, true);
       window.removeEventListener("resize", restore);
+      window.removeEventListener("blur", restore);
       restore();
     };
   }, [open, restore]);
 
+  // A control unmounted under a still cursor fires no pointerout: without this
+  // its bubble stays up over whatever replaced it. The same goes for a title
+  // re-rendered under it — the bubble would keep saying the old text, so it
+  // hides and `restore` keeps the new attribute.
+  useEffect(() => {
+    if (!tip) return;
+    const watch = new MutationObserver((records) => {
+      const el = parked.current;
+      if (!el) return;
+      if (
+        !el.isConnected ||
+        records.some((r) => r.type === "attributes" && el.hasAttribute("title"))
+      )
+        restore();
+    });
+    watch.observe(document.body, { childList: true, subtree: true });
+    if (parked.current) watch.observe(parked.current, { attributeFilter: ["title"] });
+    return () => watch.disconnect();
+  }, [tip, restore]);
+
   // Placed after the first paint, when the bubble's real size is known —
   // measuring it is the only way to flip and clamp without guessing.
   useEffect(() => {

client/src/components/__tests__/Tooltip.test.tsx

@@ -10,6 +10,7 @@
 
 import { describe, it, expect, beforeEach, afterEach, vi } from "vitest";
 import { render, screen, fireEvent, act } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
 import { GlobalTooltip } from "../Tooltip";
 
 function setup(extra?: React.ReactNode) {
@@ -24,6 +25,24 @@ function setup(extra?: React.ReactNode) {
 
 const hover = (el: Element) => fireEvent.pointerOver(el, { bubbles: true });
 const unhover = (el: Element) => fireEvent.pointerOut(el, { bubbles: true });
+// jsdom's :focus-visible is a heuristic over the previous events and leaks
+// between tests, so the tests say which kind of focus they mean: a spy answers
+// `:focus-visible` from this flag and passes every other selector through.
+let keyboardFocus = false;
+const realMatches = Element.prototype.matches;
+beforeEach(() => {
+  keyboardFocus = false;
+  vi.spyOn(Element.prototype, "matches").mockImplementation(function (this: Element, sel) {
+    return sel === ":focus-visible"
+      ? keyboardFocus && document.activeElement === this
+      : realMatches.call(this, sel);
+  });
+});
+afterEach(() => vi.restoreAllMocks());
+const tabTo = (el: HTMLElement) => {
+  keyboardFocus = true;
+  act(() => el.focus());
+};
 
 beforeEach(() => vi.useFakeTimers({ shouldAdvanceTime: true }));
 afterEach(() => vi.useRealTimers());
@@ -63,7 +82,7 @@ describe("GlobalTooltip", () => {
     setup();
     const button = screen.getByRole("button", { name: "refresh" });
 
-    fireEvent.focusIn(button);
+    tabTo(button);
     await act(async () => {
       vi.advanceTimersByTime(0);
     });
@@ -105,4 +124,165 @@ describe("GlobalTooltip", () => {
     await settle();
     expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Refresh the working copy");
   });
+  it("keeps an icon-only control's accessible name while its title is parked", async () => {
+    setup(
+      <button title="Stage this file">
+        <svg aria-hidden="true" />
+      </button>
+    );
+    const icon = screen.getByRole("button", { name: "Stage this file" });
+    tabTo(icon);
+    await act(async () => {
+      vi.advanceTimersByTime(0);
+    });
+    expect(screen.getByTestId("global-tooltip")).toBeInTheDocument();
+    expect(icon).not.toHaveAttribute("title");
+    expect(screen.getByRole("button", { name: "Stage this file" })).toBe(icon);
+
+    fireEvent.keyDown(document, { key: "Escape" });
+    // Put back exactly as found: no aria-label left behind.
+    expect(icon).toHaveAttribute("title", "Stage this file");
+    expect(icon).not.toHaveAttribute("aria-label");
+  });
+
+  it("does not flicker while the pointer crosses the control's own children", async () => {
+    setup(
+      <button title="Two parts">
+        <span data-testid="part-a">a</span>
+        <span data-testid="part-b">b</span>
+      </button>
+    );
+    const a = screen.getByTestId("part-a");
+    const b = screen.getByTestId("part-b");
+    hover(a);
+    await settle();
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Two parts");
+
+    fireEvent.pointerOut(a, { bubbles: true, relatedTarget: b });
+    fireEvent.pointerOver(b, { bubbles: true, relatedTarget: a });
+    // Still up, with no fresh delay.
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Two parts");
+  });
+
+  it("hides when its control is removed from the page", async () => {
+    const { rerender } = render(
+      <>
+        <GlobalTooltip />
+        <button title="Going away">x</button>
+      </>
+    );
+    hover(screen.getByRole("button", { name: "x" }));
+    await settle();
+    expect(screen.getByTestId("global-tooltip")).toBeInTheDocument();
+
+    rerender(
+      <>
+        <GlobalTooltip />
+      </>
+    );
+    await settle();
+    expect(screen.queryByTestId("global-tooltip")).toBeNull();
+  });
+
+  it("keeps a title React changed while the bubble was up, instead of writing the stale one back", async () => {
+    const { rerender } = render(
+      <>
+        <GlobalTooltip />
+        <button title="Pull 2 commits">pull</button>
+      </>
+    );
+    const button = screen.getByRole("button", { name: "pull" });
+    hover(button);
+    await settle();
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Pull 2 commits");
+
+    // A poll flips the hint while the pointer rests on the control.
+    rerender(
+      <>
+        <GlobalTooltip />
+        <button title="Up to date">pull</button>
+      </>
+    );
+    await settle();
+    // The bubble must not keep advertising the old text.
+    expect(screen.queryByText("Pull 2 commits")).toBeNull();
+
+    unhover(button);
+    expect(button).toHaveAttribute("title", "Up to date");
+  });
+
+  it("leaves no title behind when React removed it while the bubble was up", async () => {
+    const { rerender } = render(
+      <>
+        <GlobalTooltip />
+        <button title="Pull 2 commits">pull</button>
+      </>
+    );
+    const button = screen.getByRole("button", { name: "pull" });
+    hover(button);
+    await settle();
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Pull 2 commits");
+
+    // React "removes" an attribute that is already parked: no MutationRecord.
+    rerender(
+      <>
+        <GlobalTooltip />
+        <button>pull</button>
+      </>
+    );
+    await settle();
+    unhover(button);
+    expect(button).not.toHaveAttribute("title");
+    expect(button).not.toHaveAttribute("data-tip");
+    expect(screen.queryByTestId("global-tooltip")).toBeNull();
+  });
+
+  it("does not open on the focus a mouse click gives, only on keyboard focus", async () => {
+    const user = userEvent.setup({ advanceTimers: vi.advanceTimersByTime });
+    setup(<button title="Second control">second</button>);
+    // A click is pointer focus: :focus-visible does not match.
+    const button = screen.getByRole("button", { name: "refresh" });
+
+    await user.click(button);
+    await act(async () => {
+      vi.advanceTimersByTime(0);
+    });
+    expect(screen.queryByTestId("global-tooltip")).toBeNull();
+
+    tabTo(screen.getByRole("button", { name: "second" }));
+    await act(async () => {
+      vi.advanceTimersByTime(0);
+    });
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Second control");
+  });
+
+  it("stays up when an unrelated panel scrolls, and hides when the anchor's own scroller moves", async () => {
+    render(
+      <>
+        <GlobalTooltip />
+        <div data-testid="scroller">
+          <button title="Inside the scroller">inside</button>
+        </div>
+        <div data-testid="elsewhere" />
+      </>
+    );
+    const button = screen.getByRole("button", { name: "inside" });
+    hover(button);
+    await settle();
+
+    fireEvent.scroll(screen.getByTestId("elsewhere"));
+    expect(screen.getByTestId("global-tooltip")).toHaveTextContent("Inside the scroller");
+
+    fireEvent.scroll(screen.getByTestId("scroller"));
+    expect(screen.queryByTestId("global-tooltip")).toBeNull();
+    expect(button).toHaveAttribute("title", "Inside the scroller");
+  });
+
+  it("hides when the document itself scrolls", async () => {
+    setup();
+    hover(screen.getByRole("button", { name: "refresh" }));
+    await settle();
+    fireEvent.scroll(document);
+    expect(screen.queryByTestId("global-tooltip")).toBeNull();
+  });
 });

client/src/components/lanes/AddLaneModal.tsx

@@ -27,18 +27,6 @@ import { api } from "../../lib/api";
 import type { AiRouter, CwdSuggestion } from "../../lib/api";
 import type { Lane, LaneTool, LaneWorktree } from "../../lib/types";
 
-/** Mirrors server/lib/worktree.js `slugify`, for the branch-name PREVIEW only.
- *  Nothing is sent from it: an empty branch field means "let the server name
- *  it", which is how `LANE_BRANCH_PREFIX` keeps working. */
-function slugPreview(title: string) {
-  return title
-    .toLowerCase()
-    .normalize("NFKD")
-    .replace(/[^a-z0-9]+/g, "-")
-    .replace(/^-+|-+$/g, "")
-    .slice(0, 40);
-}
-
 /** Polls the lane until the background `git worktree add` finishes (status
  *  leaves "provisioning"), or gives up after `timeoutMs`. Returns the final
  *  lane record, or `null` on timeout. */
@@ -589,12 +577,10 @@ export function AddLaneModal({
                     id="add-lane-branch"
                     value={branch}
                     onChange={(e) => setBranch(e.target.value)}
-                    // The placeholder is what the SERVER will name the branch
-                    // if this is left empty, so the default is visible without
-                    // the form having to duplicate LANE_BRANCH_PREFIX.
-                    placeholder={
-                      title.trim() ? `feat/${slugPreview(title)}` : t("addLaneBranchPlaceholder")
-                    }
+                    // Neutral on purpose: an empty field lets the SERVER name
+                    // the branch, under a LANE_BRANCH_PREFIX this form cannot
+                    // see, so any `feat/…` preview here could be wrong.
+                    placeholder={t("addLaneBranchPlaceholder")}
                     className="w-full rounded-md border border-border-light bg-surface-0 px-3 py-1.5 font-mono text-xs text-fg-primary placeholder:text-fg-muted focus:border-accent focus:outline-none"
                   />
                   <p className="mt-1 text-[10px] text-fg-muted">{t("addLaneBranchHintAuto")}</p>

client/src/components/lanes/LaneCard.tsx

@@ -55,6 +55,13 @@ function useLaneGitFacts(laneId: number): [LaneGitFacts | null, () => void] {
   // Bumped by the git action bar: a commit or a pull changes every fact on
   // this row, and waiting out the 30s poll makes the button look inert.
   const [nonce, setNonce] = useState(0);
+  // A lane switch drops the previous lane's facts during render, so nothing
+  // (the git bar's "current branch" included) shows them until the new read.
+  const [factsLane, setFactsLane] = useState(laneId);
+  if (factsLane !== laneId) {
+    setFactsLane(laneId);
+    setFacts(null);
+  }
 
   useEffect(() => {
     let alive = true;
@@ -854,7 +861,7 @@ export default function LaneCard({
                   <button
                     type="button"
                     onClick={() => onSelectLane?.(w.id)}
-                    className="truncate text-left text-blue-400 hover:underline"
+                    className="truncate text-left text-blue-700 dark:text-blue-400 hover:underline"
                     title={w.cwd}
                   >
                     #{w.id} {w.title || w.cwd} · {w.status}

client/src/components/lanes/LaneChangesPanel.tsx

@@ -59,6 +59,8 @@ const OVERVIEW_REFRESH_MS = 15_000;
 const CONTEXT_STEPS: readonly [number, ...number[]] = [3, 12, 40, 5000];
 
 const LAST_CONTEXT = CONTEXT_STEPS[CONTEXT_STEPS.length - 1]!;
+/** Tree reads "expand all" keeps in flight at once. */
+const TREE_CONCURRENCY = 4;
 
 type Mode = "overview" | "review" | "file" | "history" | "browse";
 
@@ -191,6 +193,94 @@ function IconButton({
   );
 }
 
+/** One overview row. At module scope on purpose: declared inside the panel it
+ *  was a new component type every render, so each poll and each keystroke in
+ *  the commit box remounted every row (focus dropped to <body>). */
+function ChangeRow({
+  f,
+  range,
+  busy,
+  armed,
+  onOpen,
+  onAct,
+  onArm,
+}: {
+  f: LaneChangeFile;
+  range: LaneChangeRange;
+  busy: boolean;
+  /** This row's discard has been clicked once and awaits the second click. */
+  armed: boolean;
+  onOpen: (path: string) => void;
+  onAct: (action: "stage" | "unstage" | "discard", path: string, confirm?: boolean) => void;
+  onArm: (path: string) => void;
+}) {
+  const { t } = useTranslation(["lanes"]);
+  return (
+    <li className="group flex items-baseline gap-1">
+      <button
+        type="button"
+        onClick={() => onOpen(f.path)}
+        className="flex min-w-0 flex-1 items-baseline gap-1.5 rounded px-1 py-0.5 text-left hover:bg-surface-2"
+      >
+        <span
+          className={`w-3 shrink-0 font-mono text-[11px] font-semibold ${statusClass(
+            f.status,
+            f.untracked
+          )}`}
+        >
+          {f.untracked ? "A" : f.status.trim()[0] || "M"}
+        </span>
+        <span
+          className="min-w-0 flex-1 truncate font-mono text-[11px] text-fg-primary"
+          title={f.path}
+        >
+          {f.path}
+        </span>
+        {f.conflicted && (
+          <span className="shrink-0 text-[10px] text-status-danger">{t("changes.conflicted")}</span>
+        )}
+        <Delta added={f.added} deleted={f.deleted} binary={f.binary} />
+      </button>
+      {range === "work" && (
+        <span className="flex shrink-0 items-center gap-0.5">
+          {f.staged ? (
+            <IconButton
+              label={t("changes.unstage")}
+              disabled={busy}
+              onClick={() => onAct("unstage", f.path)}
+            >
+              <Minus className="h-3 w-3" />
+            </IconButton>
+          ) : null}
+          {f.unstaged || f.untracked ? (
+            <IconButton
+              label={t("changes.stage")}
+              disabled={busy}
+              onClick={() => onAct("stage", f.path)}
+            >
+              <Plus className="h-3 w-3" />
+            </IconButton>
+          ) : null}
+          <IconButton
+            label={armed ? t("changes.discardConfirm") : t("changes.discard")}
+            disabled={busy}
+            danger
+            onClick={() => {
+              // Two clicks, always: the first arms the row, the second is the
+              // only one that reaches the server (which itself refuses the
+              // call without `confirm: true`).
+              if (armed) onAct("discard", f.path, true);
+              else onArm(f.path);
+            }}
+          >
+            <Trash2 className={`h-3 w-3 ${armed ? "text-status-danger" : ""}`} />
+          </IconButton>
+        </span>
+      )}
+    </li>
+  );
+}
+
 export function LaneChangesPanel({ laneId }: { laneId: number }) {
   const { t } = useTranslation(["lanes"]);
   const [range, setRange] = useState<LaneChangeRange>("work");
@@ -227,7 +317,21 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
   const [message, setMessage] = useState("");
   const [error, setError] = useState<string | null>(null);
 
+  // Every diff read gets a ticket; only the newest may land. A lane switch (or
+  // a mode/range change) re-issues the read, and without this a slow reply for
+  // the previous lane painted its files — and its discard buttons — over this one.
+  const loadSeq = useRef(0);
+  // The same guard for the side reads (history, tree, whole file), which only
+  // race across a lane switch: a reply for a lane no longer shown is dropped.
+  const laneNow = useRef(laneId);
+  laneNow.current = laneId;
+  const forLane =
+    <T,>(id: number, fn: (v: T) => void) =>
+    (v: T) => {
+      if (laneNow.current === id) fn(v);
+    };
   const load = useCallback(async () => {
+    const seq = ++loadSeq.current;
     setLoading(true);
     try {
       const result = await api.lanes.changes(laneId, {
@@ -236,12 +340,30 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
         ...(mode === "file" && openFile ? { path: openFile, context } : {}),
         ...(mode === "review" ? { full: true } : {}),
       });
+      if (seq !== loadSeq.current) return;
       setData(result);
       setError(null);
     } catch (err) {
+      if (seq !== loadSeq.current) return;
+      // A tracked file opened from Files that has nothing to diff: a normal
+      // state, not a fault. The whole-file view is the answer; no banner.
+      if (mode === "file" && (err as { code?: string })?.code === "ENOFILECHANGE") {
+        setData({
+          available: true,
+          range,
+          base: null,
+          baseRef: null,
+          sha,
+          context,
+          files: [],
+          truncated: false,
+        });
+        setError(null);
+        return;
+      }
       setError(err instanceof Error ? err.message : String(err));
     } finally {
-      setLoading(false);
+      if (seq === loadSeq.current) setLoading(false);
     }
   }, [laneId, range, sha, mode, openFile, context]);
 
@@ -260,7 +382,7 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     if (mode !== "history") return;
     void api.lanes
       .gitLog(laneId, { limit: 50 })
-      .then((r) => setCommits(r.commits))
+      .then(forLane(laneId, (r) => setCommits(r.commits)))
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
   }, [mode, laneId]);
 
@@ -268,7 +390,7 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     if (mode !== "browse" || query.trim() || "" in treeChildren) return;
     void api.lanes
       .gitTree(laneId, "")
-      .then((r) => setTreeChildren((prev) => ({ ...prev, "": r.entries })))
+      .then(forLane(laneId, (r) => setTreeChildren((prev) => ({ ...prev, "": r.entries }))))
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
     // `treeChildren` is the cache this fills — reading it as a dep would loop.
     // eslint-disable-next-line react-hooks/exhaustive-deps
@@ -284,36 +406,56 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     if (path in treeChildren) return;
     void api.lanes
       .gitTree(laneId, path)
-      .then((r) => setTreeChildren((prev) => ({ ...prev, [path]: r.entries })))
+      .then(forLane(laneId, (r) => setTreeChildren((prev) => ({ ...prev, [path]: r.entries }))))
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
   };
 
   const [treeBusy, setTreeBusy] = useState(false);
+  // Which lane `treeChildren` was filled for; the reset effect moves it.
+  const treeLane = useRef(laneId);
 
   // Fetches whatever level isn't cached yet, depth-first, so "expand all"
   // works even on a tree nothing has been opened in.
   const expandAllDirs = async () => {
+    // A walk is many reads long; a lane switch mid-walk must not land lane A's
+    // tree in lane B, nor start from a cache the switch has not cleared yet.
+    const lane = laneId;
     setTreeBusy(true);
-    const cache = { ...treeChildren };
+    const cache = treeLane.current === lane ? { ...treeChildren } : {};
     const toExpand = new Set(expandedDirs);
-    const walk = async (path: string) => {
+    // Level by level, at most TREE_CONCURRENCY reads in flight: one request
+    // per folder all at once is thousands on a monorepo.
+    const visit = async (path: string, next: string[]) => {
       let entries = cache[path];
       if (!entries) {
-        entries = (await api.lanes.gitTree(laneId, path)).entries;
+        entries = (await api.lanes.gitTree(lane, path)).entries;
         cache[path] = entries;
       }
-      const dirs = entries.filter((e) => e.dir);
-      for (const d of dirs) toExpand.add(d.path);
-      await Promise.all(dirs.map((d) => walk(d.path)));
+      for (const e of entries) {
+        if (!e.dir) continue;
+        toExpand.add(e.path);
+        next.push(e.path);
+      }
     };
     try {
-      await walk("");
+      for (let level = [""]; level.length && laneNow.current === lane; ) {
+        const next: string[] = [];
+        // Workers share one iterator, so each path is taken exactly once.
+        const it = level.values();
+        await Promise.all(
+          Array.from({ length: TREE_CONCURRENCY }, async () => {
+            for (const path of it) await visit(path, next);
+          })
+        );
+        level = next;
+      }
+      if (laneNow.current !== lane) return;
       setTreeChildren(cache);
       setExpandedDirs(toExpand);
     } catch (err) {
-      setError(err instanceof Error ? err.message : String(err));
+      if (laneNow.current === lane) setError(err instanceof Error ? err.message : String(err));
     } finally {
-      setTreeBusy(false);
+      if (laneNow.current === lane) setTreeBusy(false);
     }
   };
 
@@ -323,13 +465,19 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
       setHits(null);
       return;
     }
+    // `live` drops an answer that a newer query, a mode change or a lane
+    // switch has overtaken — a slow "main" must not overwrite "mainx".
+    let live = true;
     const timer = window.setTimeout(() => {
       void api.lanes
         .gitLs(laneId, q)
-        .then((r) => setHits(r.paths))
-        .catch(() => setHits([]));
+        .then((r) => live && setHits(r.paths))
+        .catch(() => live && setHits([]));
     }, 150);
-    return () => window.clearTimeout(timer);
+    return () => {
+      live = false;
+      window.clearTimeout(timer);
+    };
   }, [mode, laneId, query]);
 
   // The whole-file read is a different endpoint from the diff, so it is its
@@ -338,8 +486,10 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     if (mode !== "file" || !whole || !openFile) return;
     void api.lanes
       .gitFile(laneId, openFile, range === "commit" && sha ? sha : "work")
-      .then((r) =>
-        setText(r.available ? { path: r.path, lines: r.lines, truncated: r.truncated } : null)
+      .then(
+        forLane(laneId, (r) =>
+          setText(r.available ? { path: r.path, lines: r.lines, truncated: r.truncated } : null)
+        )
       )
       .catch((err) => setError(err instanceof Error ? err.message : String(err)));
   }, [mode, whole, openFile, laneId, range, sha]);
@@ -352,9 +502,19 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     setSha(null);
     setRange("work");
     setTreeChildren({});
+    treeLane.current = laneId;
     setExpandedDirs(new Set());
+    setTreeBusy(false);
+    setBusy(false);
+    setError(null);
     setCommits(null);
+    setData(null);
     setCollapsed(new Set());
+    // An armed discard names a PATH, not a file in a lane — carried over, one
+    // click would discard the same path in the lane just switched to.
+    setConfirmDiscard(null);
+    // Same for a half-typed commit message: it describes the OTHER lane's work.
+    setMessage("");
     returnTo.current = null;
     pendingScroll.current = null;
   }, [laneId]);
@@ -384,34 +544,41 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
     setMode("file");
   };
 
+  // An action that resolves after a lane switch belongs to the lane left: its
+  // `load` is the pre-switch one and would take a fresh ticket and paint the
+  // old lane's files here, so everything after the await is dropped.
   const act = async (action: "stage" | "unstage" | "discard", path: string, confirm?: boolean) => {
+    const lane = laneId;
     setBusy(true);
     setError(null);
     try {
-      await api.lanes.gitAction(laneId, action, { paths: [path], ...(confirm ? { confirm } : {}) });
+      await api.lanes.gitAction(lane, action, { paths: [path], ...(confirm ? { confirm } : {}) });
+      if (laneNow.current !== lane) return;
       setConfirmDiscard(null);
       await load();
     } catch (err) {
-      setError(err instanceof Error ? err.message : String(err));
+      if (laneNow.current === lane) setError(err instanceof Error ? err.message : String(err));
     } finally {
-      setBusy(false);
+      if (laneNow.current === lane) setBusy(false);
     }
   };
 
   const commitStaged = async () => {
+    const lane = laneId;
     setBusy(true);
     setError(null);
     try {
       // `only: "staged"` — the whole point of staging file by file is that the
       // commit takes the index as it stands, with no `add -A` sweeping the
       // rest of the tree in behind it.
-      await api.lanes.gitAction(laneId, "commit", { message: message.trim(), only: "staged" });
+      await api.lanes.gitAction(lane, "commit", { message: message.trim(), only: "staged" });
+      if (laneNow.current !== lane) return;
       setMessage("");
       await load();
     } catch (err) {
-      setError(err instanceof Error ? err.message : String(err));
+      if (laneNow.current === lane) setError(err instanceof Error ? err.message : String(err));
     } finally {
-      setBusy(false);
+      if (laneNow.current === lane) setBusy(false);
     }
   };
 
@@ -427,72 +594,13 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
   const stagedCount = range === "work" ? files.filter((f) => f.staged).length : 0;
   const current = mode === "file" ? files.find((f) => f.path === openFile) : undefined;
 
-  const Row = ({ f }: { f: LaneChangeFile }) => (
-    <li className="group flex items-baseline gap-1">
-      <button
-        type="button"
-        onClick={() => show(f.path)}
-        className="flex min-w-0 flex-1 items-baseline gap-1.5 rounded px-1 py-0.5 text-left hover:bg-surface-2"
-      >
-        <span
-          className={`w-3 shrink-0 font-mono text-[11px] font-semibold ${statusClass(
-            f.status,
-            f.untracked
-          )}`}
-        >
-          {f.untracked ? "A" : f.status.trim()[0] || "M"}
-        </span>
-        <span
-          className="min-w-0 flex-1 truncate font-mono text-[11px] text-fg-primary"
-          title={f.path}
-        >
-          {f.path}
-        </span>
-        {f.conflicted && (
-          <span className="shrink-0 text-[10px] text-status-danger">{t("changes.conflicted")}</span>
-        )}
-        <Delta added={f.added} deleted={f.deleted} binary={f.binary} />
-      </button>
-      {range === "work" && (
-        <span className="flex shrink-0 items-center gap-0.5">
-          {f.staged ? (
-            <IconButton
-              label={t("changes.unstage")}
-              disabled={busy}
-              onClick={() => void act("unstage", f.path)}
-            >
-              <Minus className="h-3 w-3" />
-            </IconButton>
-          ) : null}
-          {f.unstaged || f.untracked ? (
-            <IconButton
-              label={t("changes.stage")}
-              disabled={busy}
-              onClick={() => void act("stage", f.path)}
-            >
-              <Plus className="h-3 w-3" />
-            </IconButton>
-          ) : null}
-          <IconButton
-            label={confirmDiscard === f.path ? t("changes.discardConfirm") : t("changes.discard")}
-            disabled={busy}
-            danger
-            onClick={() => {
-              // Two clicks, always: the first arms the row, the second is the
-              // only one that reaches the server (which itself refuses the
-              // call without `confirm: true`).
-              if (confirmDiscard === f.path) void act("discard", f.path, true);
-              else setConfirmDiscard(f.path);
-            }}
-          >
-            <Trash2
-              className={`h-3 w-3 ${confirmDiscard === f.path ? "text-status-danger" : ""}`}
-            />
-          </IconButton>
-        </span>
-      )}
-    </li>
-  );
+  // The armed path left the list (committed, reverted, lane switched): disarm,
+  // so a later reappearance starts from the first click again.
+  useEffect(() => {
+    if (confirmDiscard && data && !files.some((f) => f.path === confirmDiscard)) {
+      setConfirmDiscard(null);
+    }
+  }, [data, files, confirmDiscard]);
 
   return (
     <div data-testid="lane-changes" className="flex min-h-0 flex-1 flex-col">
@@ -770,13 +878,22 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
           <p className="text-[11px] text-fg-muted">{t("changes.loading")}</p>
         ) : !data.available ? (
           <p className="text-[11px] text-fg-muted">{t("changes.unavailable")}</p>
-        ) : files.length === 0 ? (
+        ) : files.length === 0 && mode !== "file" ? (
           <p className="text-[11px] text-fg-muted">{t(`changes.empty.${range}`)}</p>
         ) : mode === "overview" ? (
           <>
             <ul data-testid="lane-changes-overview" className="space-y-0.5">
               {files.map((f) => (
-                <Row key={f.path} f={f} />
+                <ChangeRow
+                  key={f.path}
+                  f={f}
+                  range={range}
+                  busy={busy}
+                  armed={confirmDiscard === f.path}
+                  onOpen={show}
+                  onAct={(...a) => void act(...a)}
+                  onArm={setConfirmDiscard}
+                />
               ))}
             </ul>
             {stagedCount > 0 && (
@@ -849,6 +966,9 @@ export function LaneChangesPanel({ laneId }: { laneId: number }) {
                     tabIndex={0}
                     onClick={() => toggleCollapsed(f.path)}
                     onKeyDown={(e) => {
+                      // Only the header's own keys: Enter on the path button
+                      // inside bubbles here and must open, not fold.
+                      if (e.target !== e.currentTarget) return;
                       if (e.key === "Enter" || e.key === " ") {
                         e.preventDefault();
                         toggleCollapsed(f.path);

client/src/components/lanes/LaneDiffView.tsx

@@ -13,10 +13,15 @@
  *   - Line pairing for split view happens here, on the rows the server already
  *     numbered. The server stays the single source of what changed; this file
  *     only decides how it looks.
+ * Past `WINDOW_THRESHOLD` rows a view renders only the rows on screen (plus an
+ * overscan) between two spacers. Rows never wrap — the box scrolls sideways —
+ * so every windowed row is pinned to `ROW_PX` and the range is pure arithmetic
+ * (`visibleRange`); the content's min-width comes from the longest line, so
+ * the horizontal extent does not jump as rows come and go.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useCallback, useRef } from "react";
+import { useMemo, useRef, useState, type CSSProperties } from "react";
 import { useTranslation } from "react-i18next";
 import { canonicalLang, highlight, tokenClass } from "../../lib/highlight";
 import type { LaneChangeFile, LaneDiffHunk, LaneDiffLine } from "../../lib/types";
@@ -32,6 +37,100 @@ function boxHeight(fill: boolean) {
   return fill ? "min-h-0 flex-1" : "max-h-[60vh]";
 }
 
+/** Rows above which a view windows; at or below it renders exactly as before. */
+export const WINDOW_THRESHOLD = 500;
+/** A windowed row's pinned height — the rows' own 11px × 1.45 line box, rounded. */
+export const ROW_PX = 16;
+const OVERSCAN = 20;
+const FIXED: CSSProperties = { height: ROW_PX };
+
+/**
+ * The `[start, end)` item range to render for a scroll position. `scrollTop`
+ * is clamped to the content first, so a stale offset (content that shrank, a
+ * layout switch) still lands on real rows instead of an empty window.
+ */
+export function visibleRange(
+  scrollTop: number,
+  viewport: number,
+  rowHeight: number,
+  count: number,
+  overscan: number
+) {
+  const top = Math.min(Math.max(0, scrollTop), Math.max(0, count * rowHeight - viewport));
+  const start = Math.max(0, Math.floor(top / rowHeight) - overscan);
+  const end = Math.min(count, Math.ceil((top + viewport) / rowHeight) + overscan);
+  return { start: Math.min(start, end), end };
+}
+
+/**
+ * Windowing state for one scroller of `count` items. Off (the whole range)
+ * at or below the threshold.
+ */
+function useWindow(count: number) {
+  const [top, setTop] = useState(0);
+  const on = count > WINDOW_THRESHOLD;
+  // The offset is tracked even while windowing is off, so a diff that GROWS
+  // past the threshold (expanded context) windows where the reader is, not at
+  // row 0. A ref, so an unwindowed diff does not re-render on every scroll.
+  const lastTop = useRef(0);
+  // count 0 = no scroller here (split layout): the browser drops its offset,
+  // so coming back to unified must start from 0, not the pre-split row.
+  if (count === 0) lastTop.current = 0;
+  const [wasOn, setWasOn] = useState(on);
+  if (wasOn !== on) {
+    setWasOn(on);
+    setTop(on ? lastTop.current : 0);
+  }
+  // ponytail: the viewport is taken as the window height — never smaller than
+  // a box that lives inside the window, so it only over-renders a few dozen
+  // rows, and it needs no ResizeObserver to follow a panel resize.
+  const range = on
+    ? visibleRange(top, window.innerHeight, ROW_PX, count, OVERSCAN)
+    : { start: 0, end: count };
+  const onScroll = (el: HTMLElement) => {
+    lastTop.current = el.scrollTop;
+    if (on) setTop(el.scrollTop);
+  };
+  return { on, range, onScroll };
+}
+
+/** Columns a line occupies under `whitespace-pre` (a tab counted at its widest). */
+function longest(texts: Iterable<string>) {
+  let max = 0;
+  for (const t of texts) {
+    let n = t.length;
+    for (let i = t.indexOf("\t"); i !== -1; i = t.indexOf("\t", i + 1)) n += 7;
+    if (n > max) max = n;
+  }
+  return max;
+}
+
+/** Stands in for `rows` off-screen rows. */
+function Spacer({ rows }: { rows: number }) {
+  return rows > 0 ? <div data-diff-spacer aria-hidden style={{ height: rows * ROW_PX }} /> : null;
+}
+
+/** A windowed list: spacer, the rendered slice, spacer, at a stable width. */
+function Windowed({
+  count,
+  range,
+  minWidth,
+  children,
+}: {
+  count: number;
+  range: { start: number; end: number };
+  minWidth: string;
+  children: React.ReactNode;
+}) {
+  return (
+    <div data-diff-content style={{ minWidth }}>
+      <Spacer rows={range.start} />
+      {children}
+      <Spacer rows={count - range.end} />
+    </div>
+  );
+}
+
 /** Language tag for a path, for the tokenizer. Unknown ones render plain. */
 function langFor(filePath: string) {
   const ext = filePath.split(".").pop() || "";
@@ -86,26 +185,51 @@ function emphasis(
   };
 }
 
-/** Pair each run of removed lines with the run of added lines following it. */
-function pairs(
-  lines: LaneDiffLine[]
-): Array<{ left: LaneDiffLine | null; right: LaneDiffLine | null }> {
-  const out: Array<{ left: LaneDiffLine | null; right: LaneDiffLine | null }> = [];
+type PairRow = { left: LaneDiffLine | null; right: LaneDiffLine | null; emph: EmphPair | null };
+type EmphPair = { del: [number, number]; add: [number, number] };
+/** A hunk's split-view rows plus each paired line's emphasis, keyed by its
+ *  index in `hunk.lines` (what the unified view renders by). */
+type Analysed = { rows: PairRow[]; emph: Map<number, [number, number]> };
+
+// Keyed by the hunk object: a hunk is immutable once fetched, so both split
+// columns and every re-render of the same data share one pass. A refetch
+// brings new objects and the old entries go with them.
+const analysed = new WeakMap<LaneDiffHunk, Analysed>();
+
+/** Pair each run of removed lines with the run of added lines following it,
+ *  carrying indices so emphasis lands without searching the hunk again. */
+export function analyseHunk(hunk: LaneDiffHunk): Analysed {
+  const hit = analysed.get(hunk);
+  if (hit) return hit;
+  const lines = hunk.lines;
+  const rows: PairRow[] = [];
+  const emph = new Map<number, [number, number]>();
   for (let i = 0; i < lines.length; ) {
     const line = lines[i]!;
     if (line.type === "ctx") {
-      out.push({ left: line, right: line });
+      rows.push({ left: line, right: line, emph: null });
       i += 1;
       continue;
     }
-    const dels: LaneDiffLine[] = [];
-    const adds: LaneDiffLine[] = [];
-    while (i < lines.length && lines[i]!.type === "del") dels.push(lines[i++]!);
-    while (i < lines.length && lines[i]!.type === "add") adds.push(lines[i++]!);
-    for (let k = 0; k < Math.max(dels.length, adds.length); k += 1) {
-      out.push({ left: dels[k] ?? null, right: adds[k] ?? null });
+    const delStart = i;
+    while (i < lines.length && lines[i]!.type === "del") i += 1;
+    const addStart = i;
+    while (i < lines.length && lines[i]!.type === "add") i += 1;
+    const dels = addStart - delStart;
+    const adds = i - addStart;
+    for (let k = 0; k < Math.max(dels, adds); k += 1) {
+      const left = k < dels ? lines[delStart + k]! : null;
+      const right = k < adds ? lines[addStart + k]! : null;
+      const e = left && right ? emphasis(left.text, right.text) : null;
+      if (e) {
+        emph.set(delStart + k, e.del);
+        emph.set(addStart + k, e.add);
+      }
+      rows.push({ left, right, emph: e });
     }
   }
+  const out = { rows, emph };
+  analysed.set(hunk, out);
   return out;
 }
 
@@ -115,9 +239,26 @@ function Gutter({ n }: { n: number | null }) {
   );
 }
 
-/** A diff row. `w-max min-w-full` so the tint still spans a scrolled-open row. */
-function Row({ tint, children }: { tint: string; children: React.ReactNode }) {
-  return <div className={`flex w-max min-w-full ${tint}`}>{children}</div>;
+/** A diff row. `w-max min-w-full` so the tint still spans a scrolled-open row.
+ *  `style` pins a windowed row's height; unwindowed rows carry none. */
+function Row({
+  tint,
+  style,
+  children,
+}: {
+  tint: string;
+  style?: CSSProperties;
+  children: React.ReactNode;
+}) {
+  return (
+    <div
+      className={`flex w-max min-w-full ${tint}`}
+      style={style}
+      data-diff-item={style ? "" : undefined}
+    >
+      {children}
+    </div>
+  );
 }
 
 /** A /20 fill plus a 3px status bar at the row's left edge, so +/- rows still
@@ -135,44 +276,110 @@ function markerClass(type: LaneDiffLine["type"]) {
   return "text-fg-muted";
 }
 
-function HunkHeader({ hunk }: { hunk: LaneDiffHunk }) {
+function HunkHeader({ hunk, fixed }: { hunk: LaneDiffHunk; fixed?: boolean }) {
   const at = hunk.header.split("@@")[1];
+  // Windowed, the header is one ROW_PX row like the lines around it.
   return (
-    <div className="bg-surface-2 px-2 py-0.5 text-[10px] text-fg-muted">
+    <div
+      className={`bg-surface-2 px-2 text-[10px] text-fg-muted ${
+        fixed ? "overflow-hidden whitespace-nowrap leading-4" : "py-0.5"
+      }`}
+      style={fixed ? FIXED : undefined}
+      data-diff-item={fixed ? "" : undefined}
+    >
       {at ? `@@${at}@@` : hunk.header}
       {hunk.context ? ` ${hunk.context}` : ""}
     </div>
   );
 }
 
+function UnifiedLine({
+  line,
+  lang,
+  emph,
+  style,
+}: {
+  line: LaneDiffLine;
+  lang: string;
+  emph?: [number, number];
+  style?: CSSProperties;
+}) {
+  return (
+    <Row tint={rowTint(line.type)} style={style}>
+      <Gutter n={line.oldLine} />
+      <Gutter n={line.newLine} />
+      <span className="whitespace-pre px-2">
+        <span className={`select-none ${markerClass(line.type)}`}>
+          {line.type === "add" ? "+" : line.type === "del" ? "-" : " "}
+        </span>
+        <Code text={line.text} lang={lang} emph={emph} />
+      </span>
+    </Row>
+  );
+}
+
 function UnifiedHunk({ hunk, lang }: { hunk: LaneDiffHunk; lang: string }) {
-  const emph = new Map<number, [number, number]>();
-  for (const { left, right } of pairs(hunk.lines)) {
-    if (!left || !right || left.type !== "del" || right.type !== "add") continue;
-    const e = emphasis(left.text, right.text);
-    if (!e) continue;
-    emph.set(hunk.lines.indexOf(left), e.del);
-    emph.set(hunk.lines.indexOf(right), e.add);
-  }
+  const { emph } = analyseHunk(hunk);
   return (
     <div>
       <HunkHeader hunk={hunk} />
       {hunk.lines.map((line, i) => (
-        <Row key={i} tint={rowTint(line.type)}>
-          <Gutter n={line.oldLine} />
-          <Gutter n={line.newLine} />
-          <span className="whitespace-pre px-2">
-            <span className={`select-none ${markerClass(line.type)}`}>
-              {line.type === "add" ? "+" : line.type === "del" ? "-" : " "}
-            </span>
-            <Code text={line.text} lang={lang} emph={emph.get(i)} />
-          </span>
-        </Row>
+        <UnifiedLine key={i} line={line} lang={lang} emph={emph.get(i)} />
       ))}
     </div>
   );
 }
 
+/** One windowable item: a hunk's header (`i < 0`) or its i-th row — a line in
+ *  unified, a paired row in split. */
+type Item = { hunk: LaneDiffHunk; i: number };
+
+function flatten(hunks: LaneDiffHunk[], layout: DiffLayout): Item[] {
+  const out: Item[] = [];
+  for (const hunk of hunks) {
+    out.push({ hunk, i: -1 });
+    const n = layout === "split" ? analyseHunk(hunk).rows.length : hunk.lines.length;
+    for (let i = 0; i < n; i += 1) out.push({ hunk, i });
+  }
+  return out;
+}
+
+function longestInHunks(hunks: LaneDiffHunk[]) {
+  return longest(
+    (function* () {
+      for (const h of hunks) for (const l of h.lines) yield l.text;
+    })()
+  );
+}
+
+function SplitLine({
+  row,
+  lang,
+  side,
+  style,
+}: {
+  row: PairRow;
+  lang: string;
+  side: "left" | "right";
+  style?: CSSProperties;
+}) {
+  const line = side === "left" ? row.left : row.right;
+  return (
+    <Row tint={rowTint(line?.type ?? null)} style={style}>
+      <Gutter n={(side === "left" ? line?.oldLine : line?.newLine) ?? null} />
+      <span className="whitespace-pre px-1">
+        {line ? (
+          <Code
+            text={line.text}
+            lang={lang}
+            emph={side === "left" ? row.emph?.del : row.emph?.add}
+          />
+        ) : null}
+      </span>
+    </Row>
+  );
+}
+
 /** One side of the split view: a hunk header, then that side's rows. */
 function SplitColumn({
   hunks,
@@ -188,42 +395,103 @@ function SplitColumn({
       {hunks.map((hunk, h) => (
         <div key={`${hunk.header}:${h}`}>
           <HunkHeader hunk={hunk} />
-          {pairs(hunk.lines).map(({ left, right }, i) => {
-            const e =
-              left && right && left.type === "del" && right.type === "add"
-                ? emphasis(left.text, right.text)
-                : null;
-            const line = side === "left" ? left : right;
-            return (
-              <Row key={i} tint={rowTint(line?.type ?? null)}>
-                <Gutter n={(side === "left" ? line?.oldLine : line?.newLine) ?? null} />
-                <span className="whitespace-pre px-1">
-                  {line ? (
-                    <Code text={line.text} lang={lang} emph={side === "left" ? e?.del : e?.add} />
-                  ) : null}
-                </span>
-              </Row>
-            );
-          })}
+          {analyseHunk(hunk).rows.map((row, i) => (
+            <SplitLine key={i} row={row} lang={lang} side={side} />
+          ))}
         </div>
       ))}
     </>
   );
 }
 
-/** Both sides, each scrolling horizontally on its own but kept in step. */
+/** A windowed split column: the shared range of items, one side of each. */
+function SplitSlice({
+  items,
+  range,
+  lang,
+  side,
+}: {
+  items: Item[];
+  range: { start: number; end: number };
+  lang: string;
+  side: "left" | "right";
+}) {
+  return (
+    <>
+      {items
+        .slice(range.start, range.end)
+        .map(({ hunk, i }, k) =>
+          i < 0 ? (
+            <HunkHeader key={range.start + k} hunk={hunk} fixed />
+          ) : (
+            <SplitLine
+              key={range.start + k}
+              row={analyseHunk(hunk).rows[i]!}
+              lang={lang}
+              side={side}
+              style={FIXED}
+            />
+          )
+        )}
+    </>
+  );
+}
+
+/** `v` held inside a scroller's own extent. */
+export function clampScroll(v: number, scrollSize: number, clientSize: number) {
+  return Math.min(v, Math.max(0, scrollSize - clientSize));
+}
+
+type Axis =
+  | ["scrollLeft", "scrollWidth", "clientWidth"]
+  | ["scrollTop", "scrollHeight", "clientHeight"];
+const AXES: Axis[] = [
+  ["scrollLeft", "scrollWidth", "clientWidth"],
+  ["scrollTop", "scrollHeight", "clientHeight"],
+];
+
+/** Mirror `src`'s offset onto `dst`, clamped to what `dst` can show. */
+function follow(src: HTMLElement, dst: HTMLElement) {
+  for (const [pos, size, client] of AXES) {
+    // `src` pinned at its own end while `dst` sits past it: `src` is the
+    // narrower column echoing a clamp we just caused. Pulling `dst` back to
+    // it would snap the wide column home every time it scrolls past the
+    // narrow one's longest line.
+    if (src[pos] >= clampScroll(Infinity, src[size], src[client]) && dst[pos] > src[pos]) continue;
+    const target = clampScroll(src[pos], dst[size], dst[client]);
+    if (dst[pos] !== target) dst[pos] = target;
+  }
+}
+
+/** Both sides, each its own scroller but kept in step on both axes. */
 function SplitView({ hunks, lang, fill }: { hunks: LaneDiffHunk[]; lang: string; fill: boolean }) {
   const left = useRef<HTMLDivElement>(null);
   const right = useRef<HTMLDivElement>(null);
+  const items = useMemo(() => flatten(hunks, "split"), [hunks]);
+  const minWidth = useMemo(() => `calc(${longestInHunks(hunks)}ch + 3rem)`, [hunks]);
+  const win = useWindow(items.length);
 
   // Mirroring fires the other pane's own scroll handler, which mirrors back —
-  // the equality check is what ends that, so no re-entry flag is needed.
-  const sync = useCallback((from: "left" | "right") => {
+  // the equality check (and `follow`'s edge rule) is what ends that, so no
+  // re-entry flag is needed. Both axes: the columns hold paired rows, so a
+  // vertical drift puts a deletion beside some unrelated line. Both columns
+  // window off the same offset, so they always hold the same row range.
+  const sync = (from: "left" | "right") => {
     const src = from === "left" ? left.current : right.current;
     const dst = from === "left" ? right.current : left.current;
-    if (!src || !dst || dst.scrollLeft === src.scrollLeft) return;
-    dst.scrollLeft = src.scrollLeft;
-  }, []);
+    if (!src || !dst) return;
+    follow(src, dst);
+    win.onScroll(src);
+  };
+
+  const column = (side: "left" | "right") =>
+    win.on ? (
+      <Windowed count={items.length} range={win.range} minWidth={minWidth}>
+        <SplitSlice items={items} range={win.range} lang={lang} side={side} />
+      </Windowed>
+    ) : (
+      <SplitColumn hunks={hunks} lang={lang} side={side} />
+    );
 
   return (
     <div className={`flex ${boxHeight(fill)}`}>
@@ -233,7 +501,7 @@ function SplitView({ hunks, lang, fill }: { hunks: LaneDiffHunk[]; lang: string;
         onScroll={() => sync("left")}
         className="w-1/2 min-w-0 overflow-auto"
       >
-        <SplitColumn hunks={hunks} lang={lang} side="left" />
+        {column("left")}
       </div>
       <div
         ref={right}
@@ -241,7 +509,7 @@ function SplitView({ hunks, lang, fill }: { hunks: LaneDiffHunk[]; lang: string;
         onScroll={() => sync("right")}
         className="w-1/2 min-w-0 overflow-auto border-l border-border-light"
       >
-        <SplitColumn hunks={hunks} lang={lang} side="right" />
+        {column("right")}
       </div>
     </div>
   );
@@ -260,6 +528,15 @@ export function LaneDiffView({
 }) {
   const { t } = useTranslation(["lanes"]);
   const lang = langFor(file.path);
+  const hunks = file.hunks;
+  const items = useMemo(() => (hunks ? flatten(hunks, "unified") : []), [hunks]);
+  const minWidth = useMemo(
+    // two gutters + the row's px-2, plus the marker column
+    () => `calc(${hunks ? longestInHunks(hunks) + 1 : 0}ch + 6rem)`,
+    [hunks]
+  );
+  // Unified only; split windows inside its own two scrollers.
+  const win = useWindow(layout === "unified" ? items.length : 0);
 
   if (file.binary)
     return <p className="px-2 py-1 text-[10px] text-fg-muted">{t("changes.binary")}</p>;
@@ -270,12 +547,31 @@ export function LaneDiffView({
     <div
       data-testid="lane-diff-view"
       data-layout={layout}
+      onScroll={(e) => win.onScroll(e.currentTarget)}
       className={`flex flex-col font-mono text-[11px] leading-[1.45] ${
         layout === "split" ? boxHeight(fill) : `overflow-auto ${boxHeight(fill)}`
       }`}
     >
       {layout === "split" ? (
         <SplitView hunks={file.hunks} lang={lang} fill={fill} />
+      ) : win.on ? (
+        <Windowed count={items.length} range={win.range} minWidth={minWidth}>
+          {items
+            .slice(win.range.start, win.range.end)
+            .map(({ hunk, i }, k) =>
+              i < 0 ? (
+                <HunkHeader key={win.range.start + k} hunk={hunk} fixed />
+              ) : (
+                <UnifiedLine
+                  key={win.range.start + k}
+                  line={hunk.lines[i]!}
+                  lang={lang}
+                  emph={analyseHunk(hunk).emph.get(i)}
+                  style={FIXED}
+                />
+              )
+            )}
+        </Windowed>
       ) : (
         file.hunks.map((hunk, i) => (
           <UnifiedHunk key={`${hunk.header}:${i}`} hunk={hunk} lang={lang} />
@@ -303,19 +599,32 @@ export function LaneFileView({
 }) {
   const { t } = useTranslation(["lanes"]);
   const lang = langFor(path);
+  const win = useWindow(lines.length);
+  // gutter + the row's px-2
+  const minWidth = useMemo(() => `calc(${longest(lines)}ch + 3.5rem)`, [lines]);
+  const row = (text: string, i: number, style?: CSSProperties) => (
+    <Row key={i} tint="" style={style}>
+      <Gutter n={i + 1} />
+      <span className="whitespace-pre px-2">
+        <Code text={text} lang={lang} />
+      </span>
+    </Row>
+  );
   return (
     <div
       data-testid="lane-file-view"
+      onScroll={(e) => win.onScroll(e.currentTarget)}
       className={`overflow-auto font-mono text-[11px] leading-[1.45] ${boxHeight(fill)}`}
     >
-      {lines.map((text, i) => (
-        <Row key={i} tint="">
-          <Gutter n={i + 1} />
-          <span className="whitespace-pre px-2">
-            <Code text={text} lang={lang} />
-          </span>
-        </Row>
-      ))}
+      {win.on ? (
+        <Windowed count={lines.length} range={win.range} minWidth={minWidth}>
+          {lines
+            .slice(win.range.start, win.range.end)
+            .map((text, k) => row(text, win.range.start + k, FIXED))}
+        </Windowed>
+      ) : (
+        lines.map((text, i) => row(text, i))
+      )}
       {truncated && (
         <p className="px-2 py-1 text-[10px] text-status-warning">{t("changes.fileTruncated")}</p>
       )}

client/src/components/lanes/LaneGitBar.tsx

@@ -15,7 +15,7 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useState } from "react";
+import { useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import {
   ArrowDown,
@@ -86,6 +86,29 @@ export function LaneGitBar({
   const [worktrees, setWorktrees] = useState<LaneWorktree[]>([]);
   const [picked, setPicked] = useState("");
   const [newBranch, setNewBranch] = useState("");
+  // LaneCard is not keyed by lane, so this bar outlives a lane switch. All of
+  // the state above describes ONE lane — a message typed for lane A, Entered
+  // after switching, would commit B's whole tree under it — so it is reset
+  // during render the moment the id changes, before anything can act on it.
+  const [shownLane, setShownLane] = useState(laneId);
+  // One action at a time, checked synchronously: `busy` is state, and a second
+  // Enter can land before the render that disables anything.
+  const inFlight = useRef(false);
+  if (shownLane !== laneId) {
+    setShownLane(laneId);
+    setBusy(null);
+    setError(null);
+    setCommitting(false);
+    setMessage("");
+    setSwitching(false);
+    setBranches(null);
+    setWorktrees([]);
+    setPicked("");
+    setNewBranch("");
+    inFlight.current = false;
+  }
+  const laneNow = useRef(laneId);
+  laneNow.current = laneId;
 
   if (!git.available) return null;
   const dirty = git.dirty + git.untracked;
@@ -94,17 +117,27 @@ export function LaneGitBar({
     action: Action,
     body?: { message?: string; only?: "staged"; branch?: string }
   ) => {
+    if (inFlight.current) return;
+    inFlight.current = true;
+    const lane = laneId;
     setBusy(action);
     setError(null);
     try {
-      await api.lanes.gitAction(laneId, action, body || {});
-      setCommitting(false);
-      setMessage("");
+      await api.lanes.gitAction(lane, action, body || {});
+      // Landed after a lane switch: every write below belongs to the lane
+      // no longer shown.
+      if (laneNow.current !== lane) return;
+      // Only a landed commit consumes the message; a fetch or pull mid-typing
+      // must leave it where it was.
+      if (action === "commit") {
+        setCommitting(false);
+        setMessage("");
+      }
       // The branch list this bar is showing is stale the moment a branch is
       // created, deleted or switched to, so it is re-read rather than patched.
-      if (action === "switch") setSwitching(false);
-      else if (action.startsWith("branch-")) {
-        setNewBranch("");
+      if (action === "switch" || action.startsWith("branch-")) {
+        if (action === "switch") setSwitching(false);
+        else setNewBranch("");
         setPicked("");
         setBranches(null);
         await refreshBranches();
@@ -113,19 +146,32 @@ export function LaneGitBar({
     } catch (err) {
       // The server appends git's own stderr to the message, so this is the
       // rejection text verbatim — hence the <pre> below rather than a line.
-      setError(err instanceof Error ? err.message : String(err));
+      // ENOREMOTE is the one refusal with no git text to show: the server's
+      // sentence is English and says less than a translated one.
+      if (laneNow.current === lane)
+        setError(
+          (err as { code?: string })?.code === "ENOREMOTE"
+            ? t("gitBar.errNoRemote")
+            : err instanceof Error
+              ? err.message
+              : String(err)
+        );
     } finally {
-      setBusy(null);
+      if (laneNow.current === lane) {
+        inFlight.current = false;
+        setBusy(null);
+      }
     }
   };
 
   const refreshBranches = async () => {
     try {
       const r = await api.lanes.gitBranches(laneId);
+      if (laneNow.current !== laneId) return;
       setBranches(r.branches);
       setWorktrees(r.worktrees || []);
     } catch {
-      setBranches([]);
+      if (laneNow.current === laneId) setBranches([]);
     }
   };
 
@@ -145,7 +191,10 @@ export function LaneGitBar({
       <div className="flex flex-wrap items-center gap-1.5">
         {/* Ahead/behind first: it is the fact that decides which of the
             buttons beside it the user actually wants. */}
-        {git.upstream ? (
+        {/* Null counts with an upstream named means git could not compare
+            the two (the upstream ref is gone): claiming "in sync" there
+            would be a lie, so it reads as not pushed. */}
+        {git.upstream && git.ahead != null && git.behind != null ? (
           <span
             data-testid="lane-git-tracking"
             title={git.upstream}
@@ -223,7 +272,7 @@ export function LaneGitBar({
             value={message}
             onChange={(e) => setMessage(e.target.value)}
             onKeyDown={(e) => {
-              if (e.key === "Enter" && message.trim()) void run("commit", { message });
+              if (e.key === "Enter" && message.trim() && !busy) void run("commit", { message });
               if (e.key === "Escape") setCommitting(false);
             }}
             placeholder={t("gitBar.commitPlaceholder")}
@@ -233,7 +282,7 @@ export function LaneGitBar({
           <Btn
             label={t("gitBar.commitAll", { count: dirty })}
             busy={busy === "commit"}
-            disabled={!message.trim()}
+            disabled={!message.trim() || !!busy}
             onClick={() => void run("commit", { message })}
           />
           {/* The Changes panel's per-file staging only means something if a
@@ -243,7 +292,7 @@ export function LaneGitBar({
             label={t("gitBar.commitStaged")}
             title={t("gitBar.commitStagedHint")}
             busy={busy === "commit"}
-            disabled={!message.trim()}
+            disabled={!message.trim() || !!busy}
             onClick={() => void run("commit", { message, only: "staged" })}
           />
           <button
@@ -289,8 +338,16 @@ export function LaneGitBar({
           {onNewLane && (
             <Btn
               label={t("gitBar.newLane")}
-              title={t("gitBar.newLaneHint")}
-              disabled={!picked}
+              // A branch any worktree holds — this lane's own included — is
+              // one `addWorktree` refuses with EBRANCHBUSY; say so up front.
+              title={
+                picked && (heldBy.has(picked) || picked === git.branch)
+                  ? t("gitBar.newLaneHeld", {
+                      path: heldBy.get(picked) ?? t("gitBar.switchCurrent"),
+                    })
+                  : t("gitBar.newLaneHint")
+              }
+              disabled={!picked || heldBy.has(picked) || picked === git.branch}
               // The sibling lane is created through the normal Add-lane flow,
               // prefilled — one provisioning path, not a second one here.
               onClick={() => onNewLane(picked)}

client/src/components/lanes/__tests__/AddLaneModal.test.tsx

@@ -245,10 +245,11 @@ describe("AddLaneModal — picking an existing branch (issue #34)", () => {
     await user.type(repoField, "/Users/tester/projects/repo");
     await screen.findByLabelText("Branch to fork from");
     await user.type(screen.getByLabelText("Title"), "Auto named");
-    // The placeholder previews the name the server will pick.
+    // No `feat/…` guess: the prefix is the server's LANE_BRANCH_PREFIX, which
+    // the form cannot see, so the placeholder only says the field is optional.
     expect(screen.getByLabelText("New branch name")).toHaveAttribute(
       "placeholder",
-      "feat/auto-named"
+      "Automatic — named from the title"
     );
 
     await user.click(screen.getByRole("button", { name: "Add lane" }));

client/src/components/lanes/__tests__/LaneCard.test.tsx

@@ -237,6 +237,17 @@ describe("LaneCard git block", () => {
     expect(screen.queryByText(/stale\/recorded/)).toBeNull();
   });
 
+  it("drops the previous lane's facts the moment the lane changes", async () => {
+    vi.mocked(api.lanes.git).mockResolvedValueOnce(facts);
+    const { rerender } = render(<LaneCard lane={makeLane()} onAction={vi.fn()} />);
+    await screen.findByText(/feat\/rename-metric/);
+
+    // The next lane's read never lands: nothing of lane 1's git may linger.
+    vi.mocked(api.lanes.git).mockReturnValueOnce(new Promise(() => {}));
+    rerender(<LaneCard lane={makeLane({ id: 2, branch: "lane/other" })} onAction={vi.fn()} />);
+    expect(screen.queryByText(/feat\/rename-metric/)).toBeNull();
+  });
+
   it("names the files behind the counts on hover, with each one's line delta", async () => {
     vi.mocked(api.lanes.git).mockResolvedValueOnce(facts);
     render(<LaneCard lane={makeLane()} onAction={vi.fn()} />);

client/src/components/lanes/__tests__/LaneChangesPanel.test.tsx

@@ -11,7 +11,7 @@
  */
 
 import { describe, it, expect, vi, beforeEach } from "vitest";
-import { render, screen, waitFor } from "@testing-library/react";
+import { render, screen, waitFor, act } from "@testing-library/react";
 import userEvent from "@testing-library/user-event";
 import { LaneChangesPanel } from "../LaneChangesPanel";
 import { api } from "../../../lib/api";
@@ -94,12 +94,14 @@ beforeEach(() => {
       },
     ],
   });
-  vi.mocked(api.lanes.gitLs).mockResolvedValue({
-    available: true,
-    paths: ["src/main.ts"],
-    truncated: false,
-    total: 1,
-  });
+  vi.mocked(api.lanes.gitLs)
+    .mockReset()
+    .mockResolvedValue({
+      available: true,
+      paths: ["src/main.ts"],
+      truncated: false,
+      total: 1,
+    });
   vi.mocked(api.lanes.gitTree).mockResolvedValue({
     available: true,
     path: "",
@@ -229,6 +231,76 @@ describe("LaneChangesPanel", () => {
     });
   });
 
+  it("disarms a pending discard when the lane changes, so one click cannot discard in another lane", async () => {
+    const user = userEvent.setup();
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    const [discard] = screen.getAllByRole("button", { name: "Discard this file's changes" });
+    await user.click(discard!);
+
+    // Lane 8 has a file at the SAME path.
+    rerender(<LaneChangesPanel laneId={8} />);
+    await waitFor(() => expect(api.lanes.changes).toHaveBeenCalledWith(8, expect.anything()));
+    expect(
+      screen.queryByRole("button", { name: "Click again to discard — this cannot be undone" })
+    ).toBeNull();
+    const [again] = await screen.findAllByRole("button", { name: "Discard this file's changes" });
+    await user.click(again!);
+    expect(api.lanes.gitAction).not.toHaveBeenCalled();
+  });
+
+  it("disarms a pending discard once that path leaves the file list", async () => {
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    const [discard] = screen.getAllByRole("button", { name: "Discard this file's changes" });
+    await user.click(discard!);
+
+    // The file was committed/reverted elsewhere; a later reappearance must not be pre-armed.
+    const base = changes();
+    const rest = base.available ? base.files.slice(1) : [];
+    vi.mocked(api.lanes.changes).mockResolvedValueOnce(changes({ files: rest }));
+    vi.mocked(api.lanes.changes).mockResolvedValue(changes());
+    // A plain refresh (the poll's twin), not a git action — an action already disarms.
+    await user.click(screen.getByRole("button", { name: "Refresh" }));
+    await waitFor(() => expect(screen.queryByText("server/lib/git-ops.js")).toBeNull());
+    await user.click(screen.getByRole("button", { name: "Refresh" }));
+    await screen.findByText("server/lib/git-ops.js");
+    expect(
+      screen.queryByRole("button", { name: "Click again to discard — this cannot be undone" })
+    ).toBeNull();
+  });
+
+  it("ignores a late response for the lane it has already left", async () => {
+    let resolveOld: (v: LaneChanges) => void = () => {};
+    vi.mocked(api.lanes.changes).mockImplementation((id) =>
+      id === 7
+        ? new Promise<LaneChanges>((r) => (resolveOld = r))
+        : Promise.resolve(
+            changes({
+              files: [
+                {
+                  path: "lane8.txt",
+                  status: "??",
+                  untracked: true,
+                  added: 1,
+                  deleted: 0,
+                  binary: false,
+                },
+              ],
+            })
+          )
+    );
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    rerender(<LaneChangesPanel laneId={8} />);
+    expect(await screen.findByText("lane8.txt")).toBeInTheDocument();
+
+    resolveOld(changes());
+    await new Promise((r) => setTimeout(r, 20));
+    expect(screen.getByText("lane8.txt")).toBeInTheDocument();
+    expect(screen.queryByText("server/lib/git-ops.js")).toBeNull();
+  });
+
   it("opens a commit from history as a commit-range review", async () => {
     const user = userEvent.setup();
     render(<LaneChangesPanel laneId={7} />);
@@ -411,6 +483,16 @@ describe("LaneChangesPanel", () => {
     });
   });
 
+  it("drops a half-typed commit message on a lane switch", async () => {
+    const user = userEvent.setup();
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-commit");
+    await user.type(screen.getByLabelText("Commit message"), "fix: meant for lane 7");
+    rerender(<LaneChangesPanel laneId={8} />);
+    await screen.findByTestId("lane-changes-commit");
+    expect(screen.getByLabelText("Commit message")).toHaveValue("");
+  });
+
   it("hides the commit box when nothing is staged", async () => {
     vi.mocked(api.lanes.changes).mockResolvedValue(
       changes({
@@ -515,6 +597,73 @@ describe("LaneChangesPanel", () => {
     );
   });
 
+  it("opens, not folds, when Enter is pressed on the path", async () => {
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Review" }));
+    await screen.findByTestId("lane-changes-review");
+
+    screen.getByRole("button", { name: "server/lib/git-ops.js" }).focus();
+    await user.keyboard("{Enter}");
+    await waitFor(() =>
+      expect(api.lanes.changes).toHaveBeenCalledWith(7, {
+        range: "work",
+        path: "server/lib/git-ops.js",
+        context: 3,
+      })
+    );
+  });
+
+  it("drops a quick-open answer for the lane it has already left", async () => {
+    const user = userEvent.setup();
+    let answer: (v: { paths: string[] }) => void = () => {};
+    vi.mocked(api.lanes.gitLs).mockReturnValueOnce(
+      new Promise((r) => {
+        answer = r as typeof answer;
+      }) as ReturnType<typeof api.lanes.gitLs>
+    );
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    await user.type(await screen.findByLabelText("Find a file by path…"), "main");
+    await waitFor(() => expect(api.lanes.gitLs).toHaveBeenCalledWith(7, "main"));
+
+    rerender(<LaneChangesPanel laneId={8} />);
+    await act(async () => answer({ paths: ["from/lane7.ts"] }));
+    // Back in Files on lane 8, before its own search lands: nothing of lane 7.
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    expect(screen.queryByText("from/lane7.ts")).toBeNull();
+  });
+
+  it("keeps the newest quick-open answer when an older one lands after it", async () => {
+    const user = userEvent.setup();
+    let answer: (v: { paths: string[] }) => void = () => {};
+    vi.mocked(api.lanes.gitLs)
+      .mockReturnValueOnce(
+        new Promise((r) => {
+          answer = r as typeof answer;
+        }) as ReturnType<typeof api.lanes.gitLs>
+      )
+      .mockResolvedValueOnce({
+        available: true,
+        paths: ["new/mainx.ts"],
+        truncated: false,
+        total: 1,
+      });
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    const box = await screen.findByLabelText("Find a file by path…");
+    await user.type(box, "main");
+    await waitFor(() => expect(api.lanes.gitLs).toHaveBeenCalledWith(7, "main"));
+    await user.type(box, "x");
+    expect(await screen.findByText("new/mainx.ts")).toBeInTheDocument();
+
+    await act(async () => answer({ paths: ["old/main-only.ts"] }));
+    expect(screen.queryByText("old/main-only.ts")).toBeNull();
+  });
+
   it("goes back to the list it came from, at the offset it was left at", async () => {
     const user = userEvent.setup();
     render(<LaneChangesPanel laneId={7} />);
@@ -537,4 +686,152 @@ describe("LaneChangesPanel", () => {
     expect(await screen.findByTestId("lane-changes-review")).toBeInTheDocument();
     expect(screen.getByTestId("lane-changes-body").scrollTop).toBe(640);
   });
+
+  it("opens an unchanged tracked file from Files with no error banner", async () => {
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    vi.mocked(api.lanes.changes).mockRejectedValue(
+      Object.assign(new Error("file has no changes in this range"), {
+        code: "ENOFILECHANGE",
+        status: 400,
+      })
+    );
+
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    await user.type(await screen.findByLabelText("Find a file by path…"), "main");
+    await user.click(await screen.findByText("src/main.ts"));
+    expect(await screen.findByTestId("lane-file-view")).toHaveTextContent("const whole = 1;");
+
+    await user.click(screen.getByRole("button", { name: "Refresh" }));
+    await waitFor(() => expect(api.lanes.changes).toHaveBeenCalledTimes(3));
+    expect(screen.queryByRole("alert")).toBeNull();
+    expect(screen.getByTestId("lane-file-view")).toBeInTheDocument();
+  });
+
+  it("keeps the overview rows mounted across re-renders (typing in the commit box)", async () => {
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-commit");
+    const before = screen.getByText("server/lib/git-ops.js");
+    await user.type(screen.getByLabelText("Commit message"), "x");
+    expect(screen.getByText("server/lib/git-ops.js")).toBe(before);
+  });
+
+  it("expands all folders with at most four tree reads in flight", async () => {
+    const user = userEvent.setup();
+    let inFlight = 0;
+    let peak = 0;
+    vi.mocked(api.lanes.gitTree).mockClear();
+    vi.mocked(api.lanes.gitTree).mockImplementation(async (_id, path = "") => {
+      inFlight += 1;
+      peak = Math.max(peak, inFlight);
+      await new Promise((r) => setTimeout(r, 5));
+      inFlight -= 1;
+      const depth = path ? path.split("/").length : 0;
+      const entries =
+        depth >= 2
+          ? [{ name: "leaf.ts", path: `${path}/leaf.ts`, dir: false }]
+          : Array.from({ length: 5 }, (_, i) => ({
+              name: `d${i}`,
+              path: path ? `${path}/d${i}` : `d${i}`,
+              dir: true,
+            }));
+      return { available: true, path, entries, truncated: false };
+    });
+    render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    await screen.findByText("d0");
+    await user.click(screen.getByRole("button", { name: "Expand all folders" }));
+
+    await waitFor(() => expect(screen.getAllByText("leaf.ts")).toHaveLength(25));
+    // Root + 5 + 25 folders, each fetched exactly once.
+    expect(api.lanes.gitTree).toHaveBeenCalledTimes(31);
+    expect(peak).toBeLessThanOrEqual(4);
+  });
+
+  it("does not repaint the previous lane after its git action resolves post-switch", async () => {
+    const user = userEvent.setup();
+    let resolveAction: (v: { action: string; ok: true; output: string }) => void = () => {};
+    vi.mocked(api.lanes.gitAction).mockReturnValue(
+      new Promise((r) => (resolveAction = r)) as ReturnType<typeof api.lanes.gitAction>
+    );
+    vi.mocked(api.lanes.changes).mockImplementation(async (id) =>
+      id === 7
+        ? changes()
+        : changes({
+            files: [
+              {
+                path: "lane8.txt",
+                status: "??",
+                untracked: true,
+                added: 1,
+                deleted: 0,
+                binary: false,
+              },
+            ],
+          })
+    );
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    const [stage] = screen.getAllByRole("button", { name: "Stage this file" });
+    await user.click(stage!);
+
+    rerender(<LaneChangesPanel laneId={8} />);
+    expect(await screen.findByText("lane8.txt")).toBeInTheDocument();
+    const lane7Reads = vi.mocked(api.lanes.changes).mock.calls.filter(([id]) => id === 7).length;
+
+    resolveAction({ action: "stage", ok: true, output: "" });
+    await new Promise((r) => setTimeout(r, 20));
+    expect(vi.mocked(api.lanes.changes).mock.calls.filter(([id]) => id === 7)).toHaveLength(
+      lane7Reads
+    );
+    expect(screen.getByText("lane8.txt")).toBeInTheDocument();
+    expect(screen.queryByText("server/lib/git-ops.js")).toBeNull();
+  });
+
+  it("does not paint the previous lane's tree when an expand-all walk outlives a lane switch", async () => {
+    const user = userEvent.setup();
+    let resolveSrc: () => void = () => {};
+    vi.mocked(api.lanes.gitTree).mockImplementation((id, path) => {
+      if (id === 7 && path === "src")
+        return new Promise(
+          (r) =>
+            (resolveSrc = () =>
+              r({
+                available: true,
+                path: "src",
+                entries: [{ name: "main.ts", path: "src/main.ts", dir: false }],
+                truncated: false,
+              }))
+        );
+      return Promise.resolve({
+        available: true,
+        path: "",
+        entries: [
+          id === 7
+            ? { name: "src", path: "src", dir: true }
+            : { name: "lib8", path: "lib8", dir: false },
+        ],
+        truncated: false,
+      });
+    });
+    const { rerender } = render(<LaneChangesPanel laneId={7} />);
+    await screen.findByTestId("lane-changes-overview");
+    await user.click(screen.getByRole("button", { name: "Files" }));
+    await screen.findByText("src");
+    await user.click(screen.getByRole("button", { name: "Expand all folders" }));
+
+    rerender(<LaneChangesPanel laneId={8} />);
+    await user.click(await screen.findByRole("button", { name: "Files" }));
+    expect(await screen.findByText("lib8")).toBeInTheDocument();
+
+    resolveSrc();
+    await new Promise((r) => setTimeout(r, 20));
+    expect(screen.getByText("lib8")).toBeInTheDocument();
+    expect(screen.queryByText("src")).toBeNull();
+    expect(screen.queryByText("main.ts")).toBeNull();
+    expect(screen.getByRole("button", { name: "Expand all folders" })).toBeEnabled();
+  });
 });

client/src/components/lanes/__tests__/LaneDiffView.test.tsx

@@ -3,13 +3,21 @@
  * @description Pins the side-by-side renderer: two independently scrolling
  * columns (a long line must be reachable, not painted over its neighbour)
  * whose horizontal scroll is mirrored, so the two halves of a line stay
- * opposite each other.
+ * opposite each other. Also pins the clamped mirroring (a narrow column may
+ * not drag the wide one back) and the row windowing of large diffs.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 import { describe, it, expect } from "vitest";
 import { fireEvent, render, screen } from "@testing-library/react";
-import { LaneDiffView } from "../LaneDiffView";
+import {
+  LaneDiffView,
+  LaneFileView,
+  ROW_PX,
+  WINDOW_THRESHOLD,
+  analyseHunk,
+  visibleRange,
+} from "../LaneDiffView";
 import type { LaneChangeFile } from "../../../lib/types";
 
 const FILE: LaneChangeFile = {
@@ -33,6 +41,17 @@ const FILE: LaneChangeFile = {
   ],
 };
 
+/** jsdom has no layout: give a scroller the extents a browser would. */
+function layout(
+  el: HTMLElement,
+  g: { scrollWidth?: number; clientWidth?: number; scrollHeight?: number; clientHeight?: number }
+) {
+  for (const [k, v] of Object.entries(g))
+    Object.defineProperty(el, k, { value: v, configurable: true });
+}
+
+const ROOMY = { scrollWidth: 2000, clientWidth: 200, scrollHeight: 2000, clientHeight: 200 };
+
 describe("LaneDiffView — side by side", () => {
   it("gives each column its own scroller", () => {
     render(<LaneDiffView file={FILE} layout="split" />);
@@ -56,6 +75,8 @@ describe("LaneDiffView — side by side", () => {
     render(<LaneDiffView file={FILE} layout="split" />);
     const left = screen.getByTestId("lane-diff-split-left");
     const right = screen.getByTestId("lane-diff-split-right");
+    layout(left, ROOMY);
+    layout(right, ROOMY);
 
     left.scrollLeft = 320;
     fireEvent.scroll(left);
@@ -66,6 +87,55 @@ describe("LaneDiffView — side by side", () => {
     expect(left.scrollLeft).toBe(0);
   });
 
+  it("keeps the split columns on the same row vertically too", () => {
+    render(<LaneDiffView file={FILE} layout="split" />);
+    const left = screen.getByTestId("lane-diff-split-left");
+    const right = screen.getByTestId("lane-diff-split-right");
+    layout(left, ROOMY);
+    layout(right, ROOMY);
+
+    left.scrollTop = 480;
+    fireEvent.scroll(left);
+    expect(right.scrollTop).toBe(480);
+
+    right.scrollTop = 40;
+    fireEvent.scroll(right);
+    expect(left.scrollTop).toBe(40);
+  });
+
+  it("clamps the mirrored offset to the other column's own extent", () => {
+    render(<LaneDiffView file={FILE} layout="split" />);
+    const left = screen.getByTestId("lane-diff-split-left");
+    const right = screen.getByTestId("lane-diff-split-right");
+    layout(left, { scrollWidth: 500, clientWidth: 200, scrollHeight: 300, clientHeight: 200 });
+    layout(right, { scrollWidth: 1000, clientWidth: 200, scrollHeight: 300, clientHeight: 200 });
+
+    right.scrollLeft = 600;
+    right.scrollTop = 250;
+    fireEvent.scroll(right);
+    expect(left.scrollLeft).toBe(300);
+    expect(left.scrollTop).toBe(100);
+  });
+
+  it("does not let the narrow column's clamped echo drag the wide one back", () => {
+    render(<LaneDiffView file={FILE} layout="split" />);
+    const left = screen.getByTestId("lane-diff-split-left");
+    const right = screen.getByTestId("lane-diff-split-right");
+    layout(left, { scrollWidth: 500, clientWidth: 200 });
+    layout(right, { scrollWidth: 1000, clientWidth: 200 });
+
+    right.scrollLeft = 600;
+    fireEvent.scroll(right);
+    // A browser fires the narrow column's scroll for the clamped write.
+    fireEvent.scroll(left);
+    expect(right.scrollLeft).toBe(600);
+
+    // A real move of the narrow column still carries the wide one.
+    left.scrollLeft = 100;
+    fireEvent.scroll(left);
+    expect(right.scrollLeft).toBe(100);
+  });
+
   it("keeps the unified layout on one scroller", () => {
     render(<LaneDiffView file={FILE} layout="unified" />);
     expect(screen.queryByTestId("lane-diff-split-left")).toBeNull();
@@ -158,3 +228,154 @@ describe("LaneDiffView — row styling", () => {
     expect(left!.className).not.toMatch(/status-(success|danger)/);
   });
 });
+
+describe("analyseHunk", () => {
+  const hunk = {
+    header: "@@ -1,4 +1,4 @@",
+    context: "",
+    oldStart: 1,
+    newStart: 1,
+    lines: [
+      { type: "ctx" as const, oldLine: 1, newLine: 1, text: "keep" },
+      { type: "del" as const, oldLine: 2, newLine: null, text: "foo = 1" },
+      { type: "add" as const, oldLine: null, newLine: 2, text: "foo = 2" },
+      { type: "del" as const, oldLine: 3, newLine: null, text: "bar = 1" },
+      { type: "add" as const, oldLine: null, newLine: 3, text: "bar = 2" },
+    ],
+  };
+
+  it("pairs each del/add with its emphasis, keyed by line index", () => {
+    const a = analyseHunk(hunk);
+    expect(a.rows).toHaveLength(3);
+    expect(a.emph.get(1)).toEqual([6, 7]);
+    expect(a.emph.get(2)).toEqual([6, 7]);
+    expect(a.emph.get(3)).toEqual([6, 7]);
+    expect(a.emph.get(4)).toEqual([6, 7]);
+    expect(a.emph.has(0)).toBe(false);
+  });
+
+  it("computes once per hunk, so both split columns and re-renders share it", () => {
+    expect(analyseHunk(hunk)).toBe(analyseHunk(hunk));
+  });
+});
+
+describe("visibleRange", () => {
+  it("covers the viewport plus overscan on both sides", () => {
+    expect(visibleRange(0, 160, 16, 1000, 5)).toEqual({ start: 0, end: 15 });
+    expect(visibleRange(1600, 160, 16, 1000, 5)).toEqual({ start: 95, end: 115 });
+  });
+
+  it("clamps a scrollTop past the content to the last full viewport", () => {
+    expect(visibleRange(99999, 160, 16, 1000, 5)).toEqual({ start: 985, end: 1000 });
+    expect(visibleRange(-50, 160, 16, 1000, 5)).toEqual({ start: 0, end: 15 });
+  });
+
+  it("never runs past a short list", () => {
+    expect(visibleRange(0, 800, 16, 3, 5)).toEqual({ start: 0, end: 3 });
+    expect(visibleRange(0, 800, 16, 0, 5)).toEqual({ start: 0, end: 0 });
+  });
+});
+
+function bigFile(n: number): LaneChangeFile {
+  const lines = Array.from({ length: n }, (_, i) =>
+    i % 2
+      ? { type: "add" as const, oldLine: null, newLine: i + 1, text: `line ${i};` }
+      : { type: "ctx" as const, oldLine: i + 1, newLine: i + 1, text: `line ${i};` }
+  );
+  lines[7] = { type: "ctx", oldLine: 8, newLine: 8, text: `line 7; ${"w".repeat(300)}` };
+  return fileWith(lines);
+}
+
+const rowsIn = (el: HTMLElement) => [...el.querySelectorAll<HTMLElement>("div.flex.w-max")];
+
+describe("LaneDiffView — windowing", () => {
+  it("renders every row below the threshold, with no spacers", () => {
+    const { container } = render(
+      <LaneDiffView file={bigFile(WINDOW_THRESHOLD - 10)} layout="unified" />
+    );
+    expect(rowsIn(container)).toHaveLength(WINDOW_THRESHOLD - 10);
+    expect(container.querySelector("[data-diff-spacer]")).toBeNull();
+  });
+
+  it("renders far fewer rows than a 5000-line diff holds, keeping the full scroll height", () => {
+    const { container } = render(<LaneDiffView file={bigFile(5000)} layout="unified" />);
+    const rows = rowsIn(container);
+    expect(rows.length).toBeGreaterThan(0);
+    expect(rows.length).toBeLessThan(300);
+    const spacers = [...container.querySelectorAll<HTMLElement>("[data-diff-spacer]")];
+    const spaced = spacers.reduce((sum, s) => sum + parseFloat(s.style.height), 0);
+    // header + 5000 lines, every item one fixed row tall
+    const rendered = container.querySelectorAll("[data-diff-item]").length;
+    expect(spaced + rendered * ROW_PX).toBe(5001 * ROW_PX);
+  });
+
+  it("moves the window with the scroll", () => {
+    const { container } = render(<LaneDiffView file={bigFile(5000)} layout="unified" />);
+    const view = screen.getByTestId("lane-diff-view");
+    expect(container.textContent).not.toContain("line 3000;");
+    view.scrollTop = 3000 * ROW_PX;
+    fireEvent.scroll(view);
+    expect(container.textContent).toContain("line 3000;");
+    expect(container.textContent).not.toContain("line 10;");
+  });
+
+  it("starts windowing where the reader is when a diff grows past the threshold", () => {
+    const { container, rerender } = render(<LaneDiffView file={bigFile(400)} layout="unified" />);
+    const view = screen.getByTestId("lane-diff-view");
+    view.scrollTop = 350 * ROW_PX;
+    fireEvent.scroll(view);
+    // Expanding context replaces the file in place, same mounted view.
+    rerender(<LaneDiffView file={bigFile(600)} layout="unified" />);
+    expect(container.textContent).toContain("line 350;");
+  });
+
+  it("starts from the top again after a round trip through split", () => {
+    const { container, rerender } = render(<LaneDiffView file={bigFile(5000)} layout="unified" />);
+    const view = screen.getByTestId("lane-diff-view");
+    view.scrollTop = 3000 * ROW_PX;
+    fireEvent.scroll(view);
+    rerender(<LaneDiffView file={bigFile(5000)} layout="split" />);
+    // In split the outer box is no scroller; the browser drops its offset.
+    screen.getByTestId("lane-diff-view").scrollTop = 0;
+    rerender(<LaneDiffView file={bigFile(5000)} layout="unified" />);
+    expect(container.textContent).toContain("line 5;");
+  });
+
+  it("holds the horizontal extent at the longest line, rendered or not", () => {
+    const { container } = render(<LaneDiffView file={bigFile(5000)} layout="unified" />);
+    const view = screen.getByTestId("lane-diff-view");
+    view.scrollTop = 3000 * ROW_PX;
+    fireEvent.scroll(view);
+    const content = container.querySelector<HTMLElement>("[data-diff-content]")!;
+    // the 300-w line 7 is off screen now, and still sets the width
+    expect(content.style.minWidth).toContain(`${"line 7; ".length + 300 + 1}ch`);
+  });
+
+  it("windows both split columns over the same row range", () => {
+    render(<LaneDiffView file={bigFile(5000)} layout="split" />);
+    const left = screen.getByTestId("lane-diff-split-left");
+    const right = screen.getByTestId("lane-diff-split-right");
+    layout(left, { scrollHeight: 5001 * ROW_PX, clientHeight: 400 });
+    layout(right, { scrollHeight: 5001 * ROW_PX, clientHeight: 400 });
+    right.scrollTop = 2000 * ROW_PX;
+    fireEvent.scroll(right);
+    expect(left.scrollTop).toBe(2000 * ROW_PX);
+    const l = rowsIn(left);
+    const r = rowsIn(right);
+    expect(l.length).toBeLessThan(300);
+    expect(l).toHaveLength(r.length);
+    expect(right.textContent).toContain("line 2001;");
+    // same rows: the right column's add has its filler row on the left
+    expect(left.textContent).toContain("line 2000;");
+  });
+
+  it("windows a whole-file read too", () => {
+    const lines = Array.from({ length: 5000 }, (_, i) => `row ${i}`);
+    const { container } = render(<LaneFileView path="a.ts" lines={lines} />);
+    expect(rowsIn(container).length).toBeLessThan(300);
+    const view = screen.getByTestId("lane-file-view");
+    view.scrollTop = 4000 * ROW_PX;
+    fireEvent.scroll(view);
+    expect(container.textContent).toContain("row 4000");
+  });
+});

client/src/components/lanes/__tests__/LaneGitBar.test.tsx

@@ -120,6 +120,15 @@ describe("LaneGitBar", () => {
     );
   });
 
+  it("re-reads the branch list after a switch, so the branch just left is not held", async () => {
+    const user = userEvent.setup();
+    render(<LaneGitBar laneId={3} git={facts({ dirty: 0, untracked: 0 })} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    await user.selectOptions(await screen.findByLabelText("Switch branch…"), "development");
+    await user.click(screen.getByRole("button", { name: "Switch branch" }));
+    await waitFor(() => expect(api.lanes.gitBranches).toHaveBeenCalledTimes(2));
+  });
+
   it("creates a branch without switching, then offers a sibling lane on it", async () => {
     const user = userEvent.setup();
     const onNewLane = vi.fn();
@@ -146,6 +155,34 @@ describe("LaneGitBar", () => {
     expect(onNewLane).toHaveBeenCalledWith("development");
   });
 
+  it("offers a sibling lane only on a branch no worktree holds — the server refuses the rest", async () => {
+    vi.mocked(api.lanes.gitBranches).mockResolvedValue({
+      available: true,
+      branches: ["development", "feat/thing", "feat/free"],
+      current: "feat/thing",
+      worktrees: [
+        { path: "/repo/lane3", branch: "feat/thing" },
+        { path: "/repo/dev", branch: "development" },
+      ],
+    });
+    const user = userEvent.setup();
+    render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} onNewLane={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    const select = await screen.findByLabelText("Switch branch…");
+    const newLane = () => screen.getByRole("button", { name: "New lane here" });
+
+    // This lane's own branch: its worktree is this lane.
+    await user.selectOptions(select, "feat/thing");
+    expect(newLane()).toBeDisabled();
+    // Held by another worktree: git allows one worktree per branch (EBRANCHBUSY).
+    await user.selectOptions(select, "development");
+    expect(newLane()).toBeDisabled();
+    expect(newLane()).toHaveAttribute("title", expect.stringContaining("/repo/dev"));
+
+    await user.selectOptions(select, "feat/free");
+    expect(newLane()).toBeEnabled();
+  });
+
   it("commits only the index when asked", async () => {
     const user = userEvent.setup();
     render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
@@ -172,4 +209,115 @@ describe("LaneGitBar", () => {
     await user.click(screen.getByRole("button", { name: /Push/ }));
     expect(await screen.findByTestId("lane-git-bar-error")).toHaveTextContent("non-fast-forward");
   });
+
+  it("keeps a half-typed commit message through a fetch", async () => {
+    const user = userEvent.setup();
+    render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /^Commit$/ }));
+    await user.type(screen.getByLabelText("Commit message"), "fix: half");
+
+    await user.click(screen.getByRole("button", { name: /Fetch/ }));
+    await waitFor(() => expect(api.lanes.gitAction).toHaveBeenCalledWith(3, "fetch", {}));
+    expect(screen.getByLabelText("Commit message")).toHaveValue("fix: half");
+  });
+
+  it("clears and closes the commit box once the commit lands", async () => {
+    const user = userEvent.setup();
+    render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /^Commit$/ }));
+    await user.type(screen.getByLabelText("Commit message"), "fix: done");
+    await user.click(screen.getByRole("button", { name: /Commit 3 files/ }));
+    await waitFor(() => expect(screen.queryByLabelText("Commit message")).toBeNull());
+  });
+
+  it("drops the commit box and message on a lane switch, so Enter cannot commit lane B under lane A's message", async () => {
+    const user = userEvent.setup();
+    const { rerender } = render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /^Commit$/ }));
+    await user.type(screen.getByLabelText("Commit message"), "fix: lane three");
+
+    rerender(<LaneGitBar laneId={4} git={facts()} onChanged={vi.fn()} />);
+    expect(screen.queryByLabelText("Commit message")).toBeNull();
+    await user.click(screen.getByRole("button", { name: /^Commit$/ }));
+    expect(screen.getByLabelText("Commit message")).toHaveValue("");
+    await user.keyboard("{Enter}");
+    expect(api.lanes.gitAction).not.toHaveBeenCalled();
+  });
+
+  it("re-reads the branch list for the new lane instead of showing the old one's", async () => {
+    const user = userEvent.setup();
+    const { rerender } = render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    await user.selectOptions(await screen.findByLabelText("Switch branch…"), "development");
+
+    rerender(<LaneGitBar laneId={4} git={facts()} onChanged={vi.fn()} />);
+    expect(screen.queryByLabelText("Switch branch…")).toBeNull();
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    await waitFor(() => expect(api.lanes.gitBranches).toHaveBeenCalledWith(4));
+    expect(await screen.findByLabelText("Switch branch…")).toHaveValue("");
+  });
+
+  it("sends one commit for a double Enter", async () => {
+    vi.mocked(api.lanes.gitAction).mockReturnValue(new Promise(() => {}));
+    const user = userEvent.setup();
+    render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /^Commit$/ }));
+    await user.type(screen.getByLabelText("Commit message"), "fix: once{Enter}{Enter}");
+    expect(api.lanes.gitAction).toHaveBeenCalledTimes(1);
+    expect(screen.getByRole("button", { name: "Commit staged" })).toBeDisabled();
+  });
+
+  it("forgets which branches the previous lane's worktrees held once the lane changes", async () => {
+    let resolveOld: (v: Awaited<ReturnType<typeof api.lanes.gitBranches>>) => void = () => {};
+    vi.mocked(api.lanes.gitBranches).mockReturnValueOnce(
+      new Promise((r) => {
+        resolveOld = r;
+      })
+    );
+    const user = userEvent.setup();
+    const { rerender } = render(
+      <LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} onNewLane={vi.fn()} />
+    );
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+
+    rerender(<LaneGitBar laneId={4} git={facts()} onChanged={vi.fn()} onNewLane={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    const select = await screen.findByLabelText("Switch branch…");
+    await waitFor(() => expect(api.lanes.gitBranches).toHaveBeenCalledWith(4));
+    // Lane 3's answer lands late: its worktree held `development`.
+    resolveOld({
+      available: true,
+      branches: ["development", "feat/thing"],
+      current: "feat/thing",
+      worktrees: [{ path: "/repo/dev", branch: "development" }],
+    });
+    await user.selectOptions(select, "development");
+    const newLane = screen.getByRole("button", { name: "New lane here" });
+    expect(newLane).toBeEnabled();
+    expect(newLane).not.toHaveAttribute("title", expect.stringContaining("/repo/dev"));
+  });
+
+  it("claims no sync state when the upstream comparison could not be counted", () => {
+    render(
+      <LaneGitBar
+        laneId={3}
+        git={facts({ upstream: "origin/gone", ahead: null, behind: null })}
+        onChanged={vi.fn()}
+      />
+    );
+    expect(screen.queryByText("in sync")).toBeNull();
+    expect(screen.queryByTestId("lane-git-tracking")).toBeNull();
+  });
+
+  it("explains a push with nowhere to go instead of the server's generic text", async () => {
+    const user = userEvent.setup();
+    vi.mocked(api.lanes.gitAction).mockRejectedValue(
+      Object.assign(new Error("this repository has no remote to push to"), { code: "ENOREMOTE" })
+    );
+    render(<LaneGitBar laneId={3} git={facts()} onChanged={vi.fn()} />);
+    await user.click(screen.getByRole("button", { name: /Push/ }));
+    expect(await screen.findByTestId("lane-git-bar-error")).toHaveTextContent(
+      "No remote to push to or pull from"
+    );
+  });
 });

client/src/i18n/locales/en/lanes.json

@@ -25,13 +25,13 @@
   "addLaneBranchBusy": "{{branch}} — already checked out at {{path}}",
   "addLaneBranchExistingHint": "The worktree checks this branch out as it is — no new branch, no base.",
   "addLaneBranchExistingLabel": "Branch to check out",
-  "addLaneBranchHintAuto": "Leave empty to use the default name shown above.",
+  "addLaneBranchHintAuto": "Leave empty to let the server name the branch from the title.",
   "addLaneBranchLabel": "New branch name",
   "addLaneBranchModeCurrent": "Current branch",
   "addLaneBranchModeCurrentHint": "Work straight on this repo's checked-out branch (main) — no worktree, no fork. Switches to Repo mode.",
   "addLaneBranchModeExisting": "Existing branch",
   "addLaneBranchModeNew": "New branch",
-  "addLaneBranchPlaceholder": "feat/my-feature",
+  "addLaneBranchPlaceholder": "Automatic — named from the title",
   "addLaneNoBranches": "This repo has no commits yet — the worktree will start empty.",
   "addLaneNoFreeBranches": "Every branch in this repo is already checked out in a worktree. Create a new branch instead.",
   "addLaneExists": "This folder already has a lane: #{{id}} {{title}}. It is selected; pick another folder to add a new one.",
@@ -180,8 +180,10 @@
   "gitBar.commitStagedHint": "Commit only what is staged in the Changes panel",
   "gitBar.fetch": "Fetch",
   "gitBar.fetchHint": "git fetch --all --prune",
+  "gitBar.errNoRemote": "No remote to push to or pull from. Add one, or set this branch's upstream, in a terminal.",
   "gitBar.inSync": "in sync",
   "gitBar.newLane": "New lane here",
+  "gitBar.newLaneHeld": "Already checked out at {{path}} — git allows one worktree per branch",
   "gitBar.newLaneHint": "Open Add lane for a sibling worktree on the selected branch",
   "gitBar.noUpstream": "not pushed",
   "gitBar.pull": "Pull",

client/src/i18n/locales/vi/lanes.json

@@ -25,13 +25,13 @@
   "addLaneBranchBusy": "{{branch}} — đang được checkout tại {{path}}",
   "addLaneBranchExistingHint": "Worktree checkout đúng nhánh này — không tạo nhánh mới, không cần nhánh gốc.",
   "addLaneBranchExistingLabel": "Nhánh cần checkout",
-  "addLaneBranchHintAuto": "Để trống để dùng tên mặc định hiển thị ở trên.",
+  "addLaneBranchHintAuto": "Để trống để server tự đặt tên nhánh theo tiêu đề.",
   "addLaneBranchLabel": "Tên nhánh mới",
   "addLaneBranchModeCurrent": "Nhánh hiện tại",
   "addLaneBranchModeCurrentHint": "Làm thẳng trên nhánh repo đang mở (main) — không tạo worktree, không fork nhánh. Chuyển sang chế độ Repo.",
   "addLaneBranchModeExisting": "Nhánh có sẵn",
   "addLaneBranchModeNew": "Nhánh mới",
-  "addLaneBranchPlaceholder": "feat/tinh-nang-cua-toi",
+  "addLaneBranchPlaceholder": "Tự động — đặt theo tiêu đề",
   "addLaneNoBranches": "Repo này chưa có commit nào — worktree sẽ bắt đầu trống.",
   "addLaneNoFreeBranches": "Mọi nhánh của repo này đều đang nằm trong một worktree. Hãy tạo nhánh mới.",
   "addLaneExists": "Thư mục này đã có lane: #{{id}} {{title}}. Lane đó đã được chọn; hãy chọn thư mục khác để thêm lane mới.",
@@ -180,8 +180,10 @@
   "gitBar.commitStagedHint": "Chỉ commit những file đã stage ở thẻ Changes",
   "gitBar.fetch": "Fetch",
   "gitBar.fetchHint": "git fetch --all --prune",
+  "gitBar.errNoRemote": "Không có remote để push hoặc pull. Hãy thêm remote, hoặc đặt upstream cho nhánh này, trong terminal.",
   "gitBar.inSync": "đã đồng bộ",
   "gitBar.newLane": "Tạo lane mới",
+  "gitBar.newLaneHeld": "Nhánh đã được checkout tại {{path}} — git chỉ cho một worktree trên mỗi nhánh",
   "gitBar.newLaneHint": "Mở Add lane để tạo worktree anh em trên nhánh đang chọn",
   "gitBar.noUpstream": "chưa push",
   "gitBar.pull": "Pull",