Repro steps

Launch a Flutter Windows app with multiple windows. Overlap the windows and move your pointer back and forth between the two windows. This will eventually crash.

This is known to affect Windows but might affect macOS and Linux too.

Root cause

Pointer events have a "device ID" that must be globally unique, or an assertion will crash the app.

Flutter Windows reuses the same "device ID" if a mouse is moved from one view to another. Usually, Windows generates a "remove pointer" event from the source view and an "add pointer" event to the destination view. However, sometimes the "remove pointer" event isn't generated when the mouse exits a view; this crashes the app on the "pointer add" event.

Possible causes:

1. win32 sometimes sends "remove" and "add" pointer events out-of-order
2. win32 sometimes reports that a mouse is on multiple HWNDs at once.