desktop_drop: fix use-after-free when posting Windows platform tasks - #526

Problem

FlutterDesktopEnginePostPlatformThreadTask in current stable Flutter (checked 3.47.7) builds its task context from a temporary:

auto context = std::make_shared<Context>(Context{callback, on_cancel, user_data});

The temporary's destructor sees invoked == false and calls on_cancel immediately. Our on_cancel deletes the queued std::function, so when the task runs, the callback reads freed memory and then deletes it a second time.

This happens on every FfiTransport::Post on Windows: every Dart -> native call (including initialize when a DropTarget mounts) and every native -> Dart reply. desktop_drop 0.9.0 crashes on Windows at startup.

Upstream fixed this in flutter/flutter#193412 (constructs Context in place and disallows copies). It is not in the current stable release yet.

Fix

Pass nullptr as on_cancel and let the callback own the task. This works on both the affected and the fixed engines. The trade-off: a task the engine discards at shutdown leaks instead of being freed. desktop_webview_window uses the same workaround.

Verification (Windows, Flutter 3.47.7, Debug)

• flutter test: 23 tests passed.
• example Debug build succeeded.
• Launched the example on the interactive desktop:
  • origin/main: crashes right after startup, 0xC0000005 in desktop_drop_plugin.dll (APPCRASH event 1000).
  • This branch: still running after 20s, no crash events.

Not covered: real drag-and-drop on Windows (no Windows integration scenarios exist for this package), and the engine-shutdown path.