[desktop_multi_window] Handle leak (~3/cycle) when a sub-window is repeatedly destroyed and recreated

Summary

When a desktop_multi_window sub-window is destroyed (e.g. it receives WM_CLOSE, or dies for any reason) and then recreated, the host process leaks ~3 Windows handles per recreate cycle (linear). There is no public API to destroy a sub-window (only show()/hide()), and the engine resources are not fully reclaimed when the window closes.

Environment

• Flutter 3.44.1 (stable), Windows 10 x64
• desktop_multi_window: 0.3.0

Observed (measured)

Windows handle count via GetProcessHandleCount, across forced-destroy + auto-recreate cycles (PostMessage WM_CLOSE to the sub-window HWND, wait for the host app to recreate it):

• Recreation succeeds 15/15, process never crashes.
• Normal operation (window kept alive, no recreation) shows no leak — handles stable (675 → 672) over 80s. So the leak is strictly tied to the destroy→recreate transition.

Analysis (from the 0.3.0 Windows source)

• WindowController (Dart) exposes only show() / hide() — there is no close() / destroy().
• The main channel mixin.one/desktop_multi_window handles only createWindow / getWindowDefinition / getAllWindows.
• FlutterWindow::OnDestroy() sets flutter_controller_ = nullptr (destroying the FlutterViewController) and calls RemoveManagedFlutterWindowLater(id). The managed FlutterWindow is only erased later via CleanupRemovedWindows(), which is invoked solely on the next Create().
• Even with that cleanup running, ~3 handles/cycle are not reclaimed — suggesting FlutterViewController/engine teardown on Windows does not fully release its thread/event handles.

Reproduction

1. WindowController.create(...) a sub-window.
2. Force-close it: PostMessage(hwnd, WM_CLOSE, 0, 0) to the sub-window's top-level HWND.
3. Recreate via WindowController.create(...).
4. Repeat. Watch GetProcessHandleCount(process) climb ~3 per cycle.

Impact

• Typical apps: negligible — sub-windows rarely die, so they're rarely recreated.
• Apps that auto-recreate a sub-window for resilience (e.g. an always-on-top overlay that respawns if it disappears) can accumulate handles over time if the window dies repeatedly.

Suggestions

1. Add an explicit WindowController.close() (Dart) + closeWindow native handler that destroys the sub-window and immediately tears down its FlutterViewController and erases it from managed_flutter_windows_, instead of deferring to the next Create().
2. Investigate whether FlutterViewController teardown itself leaks handles on Windows — if so, an upstream flutter/flutter follow-up may be warranted.

Happy to provide the full measurement harness (Win32 EnumWindows/GetProcessHandleCount/GetGuiResources) if useful.

Start with WindowController and the desktop_multi_window channel handlers for createWindow, then trace FlutterWindow::OnDestroy(), RemoveManagedFlutterWindowLater(), and CleanupRemovedWindows(). Reproduce repeated WM_CLOSE and recreation while measuring GetProcessHandleCount; done means destroyed sub-windows are fully reclaimed without linear handle growth, with coverage for the destroy-and-recreate path.