[Windows] Local engine rendering stalls without input when UI/platform threads are merged #192513

When running local builds of the windows engine the UI stalls unless there in input in the application window (touch, mouse or pen).

I've tried multiple revisions between master and a804b26.

Steps to reproduce

1. On Windows follow the instructions on setting up the engine development environment.

2. Sync dependencies gclient sync -D

3. Build the engine et build --config host_release.
   host_debug will also work but is less severe.

4. Run a Flutter Windows app using the local engine:

import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Spinny',
      theme: ThemeData.light(),
      darkTheme: ThemeData.dark(),
      home: const MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  const MyHomePage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(body: Center(child: CircularProgressIndicator()));
  }
}

flutter run -d windows --release --local-engine host_release --local-engine-host host_release

6. Let the app sit idle without moving the mouse, touching the window, or using pen input.

Expected results

The app should continue rendering frames normally while the window is idle.

Actual results

With the locally built engine, rendering only appears to progress when there is window input, such as mouse movement, touch input, or pen movement.

In release mode, the app can appear completely stalled when there is no input. Moving the mouse or otherwise generating input causes rendering to resume.

In debug/unoptimized builds, the issue is less severe but the UI appears choppy.

Additional context

Rebuilding without merged threads causes the app to render as expected.

  // main.cpp
  flutter::DartProject project(L"data");
  project.set_ui_thread_policy(flutter::UIThreadPolicy::RunOnSeparateThread);

This appears to be a latent issue in the Windows merged UI/platform thread scheduling path that is surfaced by locally building the engine with a different Windows toolchain than the official prebuilt artifacts.

The same engine revision works when using the official prebuilt Flutter 3.47.2 Windows engine artifact, but a locally built engine at the same revision stalls unless merged threads are disabled.

Windows 11 Pro 23H2
OS Build 22631.6199
MSVC 14.51.36231