When a WindowPreview is being destroyed, the class default handler for
the `destroy` signal is responsible for destroying its child actors.
This happens after the emission of the `destroy` signal, i.e. after
`WindowPreview::_onDestroy()` has been run.
The destruction of the WindowPreview's child actors now triggers a
re-pick, but due to WindowPreview having already being marked as
`CLUTTER_IN_DESTRUCTION`, it will not be picked, resulting in a `leave`
event if the cursor was on top of the WindowPreview at the time
`destroy()` was called on it.
So this leads to `WindowPreview::vfunc_leave_event()` being run after
`WindowPreview::_onDestroy()`, which means the idle started by the leave
event handler will not be removed and ends up accessing actors after
they have already been destroyed.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5512
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6065
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2738>
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