fix(terminal): preserve mouse encoding when replaying sessions (#9004)

Restore SGR and pixel mouse encoding alongside tracking modes so Copilot CLI scrolling survives switching conversations.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

packages/server/src/terminal/terminal-serializer.ts

@@ -16,13 +16,13 @@ export function createTerminalSerializer(
     // absolutely; an x==cols cursor must keep its pending wrap for the next byte.
     const position =
       buffer.cursorX < terminal.cols ? `\x1b[${buffer.cursorY + 1};${buffer.cursorX + 1}H` : "";
-    return Buffer.from(data + position + serializeCursorPresentation(terminal), "utf8");
+    return Buffer.from(data + position + serializeAdditionalModes(terminal), "utf8");
   };
 }
 
-// Stable addon 0.14 serializes buffer/modes but omits DECSCUSR and cursor visibility.
-// xterm 6 keeps the program-selected presentation in coreService, not public options.
-function serializeCursorPresentation(terminal: TerminalType): string {
+// Addon 0.14 omits mouse encoding, DECSCUSR and cursor visibility. xterm 6
+// retains these in internal services rather than the public modes/options.
+function serializeAdditionalModes(terminal: TerminalType): string {
   const core = (
     terminal as unknown as {
       _core: {
@@ -30,13 +30,17 @@ function serializeCursorPresentation(terminal: TerminalType): string {
           isCursorHidden: boolean;
           decPrivateModes: { cursorStyle?: "block" | "underline" | "bar"; cursorBlink?: boolean };
         };
+        mouseStateService: { activeEncoding: string };
       };
     }
-  )._core.coreService;
-  const visibility = `\x1b[?25${core.isCursorHidden ? "l" : "h"}`;
-  const cursorStyle = core.decPrivateModes.cursorStyle;
+  )._core;
+  let mouseEncoding = "";
+  if (core.mouseStateService.activeEncoding === "SGR") mouseEncoding = "\x1b[?1006h";
+  else if (core.mouseStateService.activeEncoding === "SGR_PIXELS") mouseEncoding = "\x1b[?1016h";
+  const visibility = `${mouseEncoding}\x1b[?25${core.coreService.isCursorHidden ? "l" : "h"}`;
+  const cursorStyle = core.coreService.decPrivateModes.cursorStyle;
   // Unset presentation belongs to each renderer's configured defaults.
   if (cursorStyle === undefined) return visibility;
   const style = { block: 2, underline: 4, bar: 6 }[cursorStyle];
-  return `${visibility}\x1b[${style - (core.decPrivateModes.cursorBlink ? 1 : 0)} q`;
+  return `${visibility}\x1b[${style - (core.coreService.decPrivateModes.cursorBlink ? 1 : 0)} q`;
 }

packages/server/src/terminal/terminal.test.ts

@@ -1702,6 +1702,52 @@ it.each([false, true])(
   },
 );
 
+it.each([
+  { name: "SGR", modes: "\x1b[?1006h", wheel: "\x1b[<64;10;5M" },
+  { name: "SGR pixels", modes: "\x1b[?1016h", wheel: "\x1b[<64;80;90M" },
+  { name: "disabled SGR", modes: "\x1b[?1006h\x1b[?1006l", wheel: "\x1b[M`*%" },
+])("preserves $name wheel reports across repeated terminal replays", async (input) => {
+  const source = new xterm.Terminal({ rows: 10, cols: 80, allowProposedApi: true });
+  const receiver = new xterm.Terminal({ rows: 10, cols: 80, allowProposedApi: true });
+  const serialize = createTerminalSerializer(source);
+  const wheel = {
+    button: 4,
+    action: 0,
+    col: 10,
+    row: 5,
+    x: 80,
+    y: 90,
+    ctrl: false,
+    alt: false,
+    shift: false,
+  };
+  // Mouse encoding is not exposed by xterm's public modes API.
+  const mouse = (
+    receiver as unknown as {
+      _core: {
+        mouseStateService: {
+          encodeMouseEvent(event: typeof wheel): string;
+        };
+      };
+    }
+  )._core.mouseStateService;
+  try {
+    await new Promise<void>((resolve) =>
+      source.write("\x1b[?1049h\x1b[?1002h" + input.modes + "TRANSCRIPT", resolve),
+    );
+    for (let replay = 0; replay < 2; replay++) {
+      receiver.reset();
+      await new Promise<void>((resolve) => receiver.write(serialize(), resolve));
+      expect(receiver.buffer.active.type).toBe("alternate");
+      expect(receiver.modes.mouseTrackingMode).toBe("drag");
+      expect(mouse.encodeMouseEvent(wheel)).toBe(input.wheel);
+    }
+  } finally {
+    receiver.dispose();
+    source.dispose();
+  }
+});
+
 it.each([
   { name: "unset", presentation: "", style: undefined, visible: true },
   {