fix(copilot): keep a drag a local selection so Auto Copy works

Copilot's TUI turns mouse tracking on itself. Under `strip-mux-only` those DECSETs reached
xterm, which then reported a plain drag to the TUI instead of selecting (only Shift+drag
selected), so Auto Copy and copy-on-select did nothing. This is the failure opencode had and
`strip-mux-and-mouse` was added for: the DECSETs are stripped, a drag is a local selection and
clicks still reach the CLI through the browser's hand-encoded tap (`cliMouseTracking`).

Stripping them also stops xterm from encoding the wheel for the CLI, so
`_shouldForwardWheelToApp` now forwards it as SGR reports while Copilot is tracking the mouse
(the same bytes xterm sent natively, which is what scrolling was verified against). Shift+wheel
stays on local scrollback and a stale flag falls back to PageUp/PageDown.

Tests: the strip flavor for copilot (stripped DECSETs, 3J kept, cliMouseTracking, direct-PTY
untouched) and the wheel gate (forwards while tracking, no other non-claude mode changes).

test/claude-scrollback-strip.test.ts

@@ -102,7 +102,7 @@ describe('isMuxAltScreenOnlyStripMode', () => {
       expect(isMuxAltScreenOnlyStripMode(mode, false)).toBe(false);
     }
     // The full strip and the mouse strip already own their modes; never double-gate.
-    for (const mode of ['claude', 'codex', 'gemini', 'opencode'] as const) {
+    for (const mode of ['claude', 'codex', 'gemini', 'opencode', 'copilot'] as const) {
       expect(isMuxAltScreenOnlyStripMode(mode, true)).toBe(false);
     }
   });
@@ -366,3 +366,35 @@ describe('strip decision table: live stream = replay, for every stock CLI', () =
     }
   }
 });
+
+/**
+ * GitHub Copilot CLI is the same shape as opencode: a full-screen TUI that turns mouse tracking
+ * on itself. Left in, xterm reports a plain drag to the TUI instead of selecting, so Auto Copy and
+ * copy-on-select did nothing (only Shift+drag selected). Same strip, same reason.
+ */
+describe('copilot: alt-screen + mouse DECSETs stripped, 3J kept', () => {
+  it('is a mouse-strip mode under tmux, and only there', () => {
+    expect(isMuxMouseStripMode('copilot', true)).toBe(true);
+    expect(isMuxMouseStripMode('copilot', false)).toBe(false);
+  });
+
+  it('drops mouse tracking so xterm keeps local text selection, and keeps 3J', () => {
+    const session = new Session({ workingDir: '/tmp', mode: 'copilot' as never, useMux: true });
+    handleOutput(session, '\x1b[?1003h\x1b[?1006h\x1b[3JTUI\x1b[?1006l\x1b[?1003l');
+    expect(session.terminalBuffer).toBe('\x1b[3JTUI');
+  });
+
+  it('publishes cliMouseTracking so the browser can forward the wheel and hand-encode clicks', () => {
+    const session = new Session({ workingDir: '/tmp', mode: 'copilot' as never, useMux: true });
+    handleOutput(session, '\x1b[?1002h\x1b[?1006hTUI');
+    expect(session.toState().cliMouseTracking).toBe(true);
+    handleOutput(session, '\x1b[?1002l\x1b[?1006l');
+    expect(session.toState().cliMouseTracking).toBeFalsy();
+  });
+
+  it('leaves a direct-PTY copilot pane untouched', () => {
+    const session = new Session({ workingDir: '/tmp', mode: 'copilot' as never, useMux: false });
+    handleOutput(session, '\x1b[?1049h\x1b[?1002hTUI');
+    expect(session.terminalBuffer).toBe('\x1b[?1049h\x1b[?1002hTUI');
+  });
+});

test/terminal-scroll-routing.test.ts

@@ -707,6 +707,29 @@ describe('pageKeysForGesture, the pure gesture rules both panes share', () => {
   });
 });
 
+describe('GitHub Copilot wheel forwarding', () => {
+  // Copilot's mouse DECSETs are stripped so a drag selects locally, which stops xterm from
+  // encoding the wheel for it; the gate must therefore forward by hand, but only while tracking.
+  it('forwards the wheel while the CLI tracks the mouse, with no version gate', () => {
+    const { app } = hollowApp({ mode: 'copilot', cliMouseTracking: true });
+    expect(app._shouldForwardWheelToApp({ shiftKey: false })).toBe(true);
+  });
+
+  it('keeps Shift+wheel on local scrollback and falls back to paging when not tracking', () => {
+    const tracking = hollowApp({ mode: 'copilot', cliMouseTracking: true });
+    expect(tracking.app._shouldForwardWheelToApp({ shiftKey: true })).toBe(false);
+    const idle = hollowApp({ mode: 'copilot', cliMouseTracking: false });
+    expect(idle.app._shouldForwardWheelToApp({ shiftKey: false })).toBe(false);
+  });
+
+  it('does not change what other non-claude modes do', () => {
+    for (const mode of ['codex', 'opencode', 'grok', 'gemini']) {
+      const { app } = hollowApp({ mode, cliMouseTracking: true });
+      expect(app._shouldForwardWheelToApp({ shiftKey: false }), mode).toBe(false);
+    }
+  });
+});
+
 describe('the paging gates asked for another pane (a TerminalTile)', () => {
   it('exports the paging math, and the primary pane runs on it', () => {
     const { app, sent, windowRef } = hollowApp();