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windowPreview: Avoid LEAVE events triggered by grab of our own StButton

Jonas Dreßler requested to merge verdre/gnome-shell:avoid-mapping-on-grab-crossing into main

commit message:

Since the ClutterGrab rework, grabbing properly emits crossing events.

StButtons take a ClutterGrab as soon as they are pressed, so pressing
the close StButton of the WindowPreview takes a grab and causes a
LEAVE+key-focus-out event getting sent to the WindowPreview. This in
turn makes us hide our overlay (which the StButton is part of). We
automatically ungrab ClutterGrabs when hiding actors, so the StButtons
grab now gets released again, key-focus and hover state are updated
again and we emit an ENTER event to the WindowPreview. The preview now
tries show its overlay again and things explode because we re-enter the
mapping machinery.

For the LEAVE event we can break this cycle by detecting the GRAB_NOTIFY
crossing event of our own StButton and not reacting to that.

We should do the same for the key-focus-out event, but these don't pass
context information like a GRAB_NOTIFY flag yet, so just check the
current grab actor here.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3165

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