fix(terminal): touchpad scrolls 1:1 with the finger

xterm.js sends one wheel report per event whatever its size and damps a
touchpad's pixel deltas x0.3, so a swipe crawled. With mouse tracking on,
TerminalView now sends the SGR reports itself (lib/wheelReports.ts): one
per 5 rows of travel (what one report scrolls in a Claude pane), the
remainder carried between events, at least one per wheel notch, at the
pointer's cell with xterm's modifier bits.

client/src/lib/__tests__/wheelReports.test.ts

@@ -0,0 +1,42 @@
+/**
+ * @file wheelReports.test.ts
+ * @description A touchpad scrolls 1:1 with the finger (one report per
+ * LINES_PER_REPORT rows, remainder carried), a wheel notch always sends one.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect } from "vitest";
+import { LINES_PER_REPORT, wheelReports, type WheelState } from "../wheelReports";
+
+const ROW = 20;
+const px = (deltaY: number) => ({ deltaY, deltaMode: 0 });
+
+describe("wheelReports", () => {
+  it("adds small touchpad steps up to one report per LINES_PER_REPORT rows", () => {
+    const s: WheelState = { partial: 0 };
+    let sent = 0;
+    // 30 steps of 10px = 300px = 15 rows.
+    for (let i = 0; i < 30; i++) sent += wheelReports(s, px(10), ROW, 40);
+    expect(sent).toBe(15 / LINES_PER_REPORT);
+  });
+
+  it("drops the carried remainder when the swipe turns around", () => {
+    const s: WheelState = { partial: 0 };
+    wheelReports(s, px(40), ROW, 40); // 2 rows down, nothing sent yet
+    let sent = 0;
+    for (let i = 0; i < 5; i++) sent += wheelReports(s, px(-20), ROW, 40); // 5 rows up
+    expect(sent).toBe(-1);
+  });
+
+  it("sends at least one report for a wheel notch, in either delta mode", () => {
+    expect(wheelReports({ partial: 0 }, px(100), ROW, 40)).toBe(1);
+    expect(wheelReports({ partial: 0 }, px(-100), ROW, 40)).toBe(-1);
+    expect(wheelReports({ partial: 0 }, { deltaY: 3, deltaMode: 1 }, ROW, 40)).toBe(1);
+    expect(wheelReports({ partial: 0 }, { deltaY: 1, deltaMode: 2 }, ROW, 40)).toBe(8);
+  });
+
+  it("sends nothing for a horizontal-only event or an unmeasured terminal", () => {
+    expect(wheelReports({ partial: 0 }, px(0), ROW, 40)).toBe(0);
+    expect(wheelReports({ partial: 0 }, px(10), 0, 40)).toBe(0);
+  });
+});

client/src/lib/wheelReports.ts

@@ -0,0 +1,59 @@
+/**
+ * @file wheelReports.ts — turns a browser wheel event into the number of SGR
+ * wheel reports a terminal pane gets, in proportion to how far the finger or
+ * the wheel moved, the way a native terminal does.
+ *
+ * xterm.js sends ONE report per wheel event whatever its size, and damps a
+ * touchpad's pixel deltas ×0.3 before it counts even that one — a touchpad
+ * swipe crawled. In our panes every report moves a fixed stretch (Claude's
+ * `CLAUDE_CODE_SCROLL_SPEED`, tmux's copy-mode `WHEEL_LINES`), so a report is
+ * sent per `LINES_PER_REPORT` rows of travel: a touchpad scrolls 1:1 with the
+ * finger, and a wheel notch always sends at least one.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+/** Rows one report scrolls in a Claude pane (server/lib/pty-run.js CLAUDE_SCROLL_SPEED). */
+export const LINES_PER_REPORT = 5;
+
+/** A pixel delta this large or larger is a wheel notch, not a touchpad step (xterm.js's own cut). */
+const NOTCH_PX = 50;
+
+const DOM_DELTA_PIXEL = 0;
+const DOM_DELTA_LINE = 1;
+const DOM_DELTA_PAGE = 2;
+
+export interface WheelState {
+  /** Touchpad travel not yet sent, in rows (signed). */
+  partial: number;
+}
+
+/**
+ * Signed report count for `ev` — negative scrolls up. `rowPx` is one row's
+ * height on screen (CSS scale included), `rows` the terminal's height.
+ */
+export function wheelReports(
+  state: WheelState,
+  ev: Pick<WheelEvent, "deltaY" | "deltaMode">,
+  rowPx: number,
+  rows: number
+): number {
+  if (!ev.deltaY || !(rowPx > 0)) return 0;
+  const sign = ev.deltaY < 0 ? -1 : 1;
+  if (ev.deltaMode === DOM_DELTA_PIXEL && Math.abs(ev.deltaY) < NOTCH_PX) {
+    // Touchpad: carry the remainder, so slow swipes still add up.
+    if (Math.sign(state.partial) === -sign) state.partial = 0; // turned around
+    state.partial += ev.deltaY / rowPx;
+    const n = Math.trunc(state.partial / LINES_PER_REPORT);
+    state.partial -= n * LINES_PER_REPORT;
+    return n;
+  }
+  // A notch (or a fast flick): at least one report, more for a long one.
+  state.partial = 0;
+  const lines =
+    ev.deltaMode === DOM_DELTA_LINE
+      ? ev.deltaY
+      : ev.deltaMode === DOM_DELTA_PAGE
+        ? ev.deltaY * rows
+        : ev.deltaY / rowPx;
+  return sign * Math.max(1, Math.round(Math.abs(lines) / LINES_PER_REPORT));
+}