On macOS the Open and Save panels ran modally on the UI thread, inside
the window's event handler. Resizing the panel delivered window events
to that handler again, and the app crashed when the mouse was released.
They now open from a thread of their own, as on Linux (#592): rfd runs
the panel on the main thread from the run loop, outside the handler, and
the app picks the answer up when it comes.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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