diff --git a/server/__tests__/tmux.test.js b/server/__tests__/tmux.test.js
index 951fc23..146bcdf 100644
--- a/server/__tests__/tmux.test.js
+++ b/server/__tests__/tmux.test.js
@@ -47,6 +47,7 @@ describe("tmux wrapper", () => {
       "--model",
       "opus",
     ]);
+    assert.deepEqual(calls[1], ["set-option", "-t", "ccam-lane-1", "mouse", "on"]);
   });
 
   it("killSession never throws when the session is already gone", () => {
diff --git a/server/lib/tmux.js b/server/lib/tmux.js
index 9f1336c..b6fe237 100644
--- a/server/lib/tmux.js
+++ b/server/lib/tmux.js
@@ -40,6 +40,11 @@ function hasSession(name) {
  */
 function newSession({ name, cwd, argv }) {
   execImpl(["new-session", "-d", "-s", name, "-c", cwd, "--", ...argv]);
+  // Without this the pane has no scrollbar/drag scroll at all — wheel events
+  // just pass through to the running program instead of entering tmux's own
+  // copy-mode scrollback. xterm.js forwards tmux's mouse-tracking escapes
+  // automatically once mouse mode is on, so no client-side change is needed.
+  execImpl(["set-option", "-t", name, "mouse", "on"]);
 }
 
 /**