fix(terminal): send SGR mouse wheel reports with the button codes app… (#15817)

* fix(terminal): send SGR mouse wheel reports with the button codes apps expect

xterm.dart 4.0.0 encodes the wheel buttons as 64+4..64+7 rather than
64+0..64+3, so the low bits land on the modifier field and every wheel
report the terminal emits reads as wheel-with-Shift. Strict full-screen
applications reject the modified event, which is why neither the mouse
wheel nor the trackpad scrolls anything once the peer application takes
over the alternate screen.

Install a mouse handler that keeps every upstream reporting decision and
only re-encodes the wheel buttons as 64..67. Non-wheel reports pass
through untouched, and the emitted bytes stay identical once upstream
ships the same fix, so this can be dropped without a behavior change.

Upstream: TerminalStudio/xterm.dart#238

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(terminal): correct the wheel report row, drop the wasted report build

Address review feedback on the wheel button fix:

- The X10/utf row was encoded as `32 + y + 1` while y is already 1-based, so
  every normal-mode report pointed one row too low and the `y > limit` guard
  disagreed with what it emitted.
- Gate the wheel path on `mouseMode.reportScroll` and the button state instead
  of building and discarding a full report string from `defaultMouseHandler`
  on every scroll tick. This also makes the hardcoded SGR 'M' provably right,
  since a wheel release now returns before the report is built.
- Derive the wire code as `id - 4` and drop `_wheelButtonId`, whose `default`
  branch was unreachable and defeated enum exhaustiveness.
- Assign `mouseHandler` after construction so the `Terminal(...)` line stays
  untouched.

Cover the utf, urxvt, null-byte overflow and click-only branches, and assert
that TerminalModel actually installs the handler.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

flutter/lib/models/terminal_model.dart

@@ -10,6 +10,7 @@ import 'package:xterm/xterm.dart';
 import 'input_modifier_utils.dart';
 import 'model.dart';
 import 'platform_model.dart';
+import 'terminal_mouse_handler.dart';
 
 class TerminalModel with ChangeNotifier {
   final String id; // peer id
@@ -129,6 +130,7 @@ class TerminalModel with ChangeNotifier {
 
   TerminalModel(this.parent, [this.terminalId = 0]) : id = parent.id {
     terminal = Terminal(maxLines: 10000);
+    terminal.mouseHandler = const WheelButtonFixMouseHandler();
     terminalController = TerminalController();
 
     // Setup terminal callbacks

flutter/lib/models/terminal_mouse_handler.dart

@@ -0,0 +1,42 @@
+import 'package:xterm/xterm.dart';
+
+/// xterm 4.0.0 encodes wheel buttons as 68..71; the extra bit reads as a Shift
+/// modifier, so strict full-screen apps ignore the report and never scroll.
+/// Upstream fix: TerminalStudio/xterm.dart#238.
+class WheelButtonFixMouseHandler implements TerminalMouseHandler {
+  const WheelButtonFixMouseHandler();
+
+  @override
+  String? call(TerminalMouseEvent event) {
+    if (!event.button.isWheel) {
+      return defaultMouseHandler(event);
+    }
+    // Same gate as UpDownMouseHandler: only the scroll modes report a wheel,
+    // and a wheel release is never reported, so the report is always a press.
+    if (!event.state.mouseMode.reportScroll ||
+        event.buttonState == TerminalMouseButtonState.up) {
+      return null;
+    }
+    return _reportWheel(event);
+  }
+
+  String _reportWheel(TerminalMouseEvent event) {
+    // Wheel buttons 4..7 go on the wire as 64..67, but `id` is 64 + 4..7.
+    final button = event.button.id - 4;
+    final x = event.position.x + 1;
+    final y = event.position.y + 1;
+    switch (event.state.mouseReportMode) {
+      case MouseReportMode.normal:
+      case MouseReportMode.utf:
+        final limit =
+            event.state.mouseReportMode == MouseReportMode.normal ? 223 : 2015;
+        final col = x > limit ? '\x00' : String.fromCharCode(32 + x);
+        final row = y > limit ? '\x00' : String.fromCharCode(32 + y);
+        return '\x1b[M${String.fromCharCode(32 + button)}$col$row';
+      case MouseReportMode.sgr:
+        return '\x1b[<$button;$x;${y}M';
+      case MouseReportMode.urxvt:
+        return '\x1b[${32 + button};$x;${y}M';
+    }
+  }
+}