If the user presses a button on a pointer device and then moves out the
Stage X11 will emit the following events:
LeaveNotify ➔ MotionNotify ... ➔ ButtonRelease ➔ LeaveNotify
The second LeaveNotify differs from the first by the state field.
Unfortunately, ClutterCrossingEvent doesn't have a modifier_state field
like other events, so we cannot provide a way for programmatically
distinguishing them from a Clutter perspective. This is also an X11-ism
we might not even want to replicate on every backend with sane
enter/leave semantics.
For this reason we should check inside the X11 event processing if the
pointer device has already left the Stage and ignore the second
LeaveNotify.
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