fix: texture render lifetime protocol, watchdog fallback, startup probe

Root cause of #15848 (and the long-standing macOS #6296 / Linux #3343
class): raw native texture pointers are shared across the platform
thread, the engine raster thread and the video thread, with teardown
ordered by a 100 ms sleep - or, when moving a tab to a new window, by
nothing at all. A lost race frees the texture while it is still in use:
the raster thread parks on a destroyed lock (frozen/black view, a
never-presented 'transparent' hole, every later session black) and the
video thread hangs while holding session locks (app half-dead until
restart, still reported as Responding).

- unregister textures with compare-and-clear (new session_unregister_*
  FFI): a late clear can no longer wipe a new window's registration
  (#8016) and Rust never keeps pushing into a freed texture; the 100 ms
  sleeps are gone (the plugins now drain in-flight pushes and defer
  object deletion until the raster thread is done)
- guard the async texture create path against destroy racing it (#13596)
- per-display locks: the per-frame plugin call no longer holds
  session-level locks, so a stalled plugin or driver call cannot freeze
  every window's UI thread
- adopt the frame size after 30 consecutive mismatches instead of
  dropping frames forever (silent black screen on a live connection)
- watchdog: frames pushed but never consumed by the engine fall the
  session back to software rendering live, record texture-render-health,
  and flip the effective default off; toggling the option clears the
  record and re-arms validation
- Dart raster-stall monitor records a hung raster thread for the next
  launch (rendering cannot be rescued in-process in that state)
- startup probe: render one frame through a 1x1 texture in the main
  window each launch; failure disables texture rendering before the
  first session goes black, a pass self-heals a stale failure record

Platform defaults are unchanged (macOS off, Win10+ on, Linux on).
Requires flutter_texture_rgba_renderer ad4c37e and
flutter_gpu_texture_renderer 767bb9f (pinned in pubspec).

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

flutter/lib/desktop/pages/desktop_home_page.dart

@@ -25,6 +25,7 @@ import 'package:url_launcher/url_launcher.dart';
 import 'package:window_manager/window_manager.dart';
 import 'package:window_size/window_size.dart' as window_size;
 import '../widgets/button.dart';
+import '../widgets/texture_render_probe.dart';
 
 class DesktopHomePage extends StatefulWidget {
   const DesktopHomePage({Key? key}) : super(key: key);
@@ -60,15 +61,20 @@ class _DesktopHomePageState extends State<DesktopHomePage>
   Widget build(BuildContext context) {
     super.build(context);
     final isIncomingOnly = bind.isIncomingOnly();
-    return _buildBlock(
-        child: Row(
-      crossAxisAlignment: CrossAxisAlignment.start,
+    return Stack(
       children: [
-        buildLeftPane(context),
-        if (!isIncomingOnly) const VerticalDivider(width: 1),
-        if (!isIncomingOnly) Expanded(child: buildRightPane(context)),
+        _buildBlock(
+            child: Row(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            buildLeftPane(context),
+            if (!isIncomingOnly) const VerticalDivider(width: 1),
+            if (!isIncomingOnly) Expanded(child: buildRightPane(context)),
+          ],
+        )),
+        const Positioned(left: 0, top: 0, child: TextureRenderProbe()),
       ],
-    ));
+    );
   }
 
   Widget _buildBlock({required Widget child}) {

flutter/lib/desktop/pages/remote_page.dart

@@ -156,6 +156,7 @@ class _RemotePageState extends State<RemotePage>
           widget.tabController?.state.listen(_onMacOSTabStateChanged);
     }
     Get.put<FFI>(_ffi, tag: widget.id);
+    _RasterStallMonitor.start();
     _ffi.imageModel.addCallbackOnFirstImage((String peerId) {
       _ffi.canvasModel.activateLocalCursor();
       showKBLayoutTypeChooserIfNeeded(
@@ -1402,3 +1403,45 @@ class CursorPaint extends StatelessWidget {
     );
   }
 }
+
+/// Detects a hung raster thread: the UI keeps scheduling frames but the
+/// engine never reports completed frame timings. Rendering cannot be rescued
+/// in-process (software rendering needs the same raster thread), so this only
+/// records the breakage — the next launch then defaults texture rendering to
+/// off and the startup probe re-validates the environment.
+class _RasterStallMonitor {
+  static bool _started = false;
+  static bool _reported = false;
+  static DateTime? _lastTimings;
+  static DateTime? _scheduledSince;
+
+  static void start() {
+    if (_started || isWeb) return;
+    _started = true;
+    SchedulerBinding.instance.addTimingsCallback((_) {
+      _lastTimings = DateTime.now();
+    });
+    Timer.periodic(const Duration(seconds: 2), (_) {
+      if (_reported) return;
+      // A minimized window legitimately stops producing frame timings.
+      if (stateGlobal.isMinimized ||
+          !SchedulerBinding.instance.hasScheduledFrame) {
+        _scheduledSince = null;
+        return;
+      }
+      final now = DateTime.now();
+      _scheduledSince ??= now;
+      final lastTimings = _lastTimings;
+      final stalled =
+          now.difference(_scheduledSince!) > const Duration(seconds: 10) &&
+              (lastTimings == null || lastTimings.isBefore(_scheduledSince!));
+      if (stalled) {
+        _reported = true;
+        bind.mainSetLocalOption(
+            key: kOptionTextureRenderHealth, value: 'failed-raster-stall');
+        debugPrint(
+            'raster thread stall detected, texture rendering disabled for next launch');
+      }
+    });
+  }
+}