There are plugin APIs that can only be called from the main thread, but there's not a particularly good way for Windows plugins to move calls to that thread AFAIK. Options include:

• Each plugin doing the hidden-window message loop implementation themselves.
• Using the top-level window proc override registration to send themselves a custom message and hope nobody else uses it in a way that interferes.

This came up in the Texture PR and the solution there was to make most of it thread-safe; applying that more broadly is another option (e.g., to event streams). But a generic way to move things to the main thread would avoid needing to make lots of thread-safe APIs.

Proposed API:

class FlutterEngine : public PluginRegistry {

  // ...

  // Schedule a callback to be called on the platform thread.
  //
  // This may be called on any thread. The callback is executed only
  // once on the platform thread.
  void PostPlatformThreadTask(std::function<void()> callback);
}

// Schedule a callback to be invoked on the engine's platform thread.
//
// This may be called on any thread. The callback is executed only
// once on the platform thread.
FLUTTER_EXPORT void FlutterDesktopEnginePostPlatformThreadTask(
    FlutterDesktopEngineRef engine,
    VoidCallback callback,
    void* user_data);

/cc @cbracken

(We would only do one of #134346 and this.)