When there are 3 or more windows in WorkspaceLayout, showing or hiding
window preview overlay in some certain orders could cause
inconsistencies in windows' vertical arrangement.
Let's take window A, B, C as an example. Initially, A is above B and B
is above C in workspace layout like this: A -> B -> C.
After opening activities, user could:
1. Move cursor to B preview, which would move B above all in layout:
B -> A -> C
2. Move cursor from B to C preview. When C's showOverlay() is called
before B's hideOverlay(), _restack() would move C above all and don't
change B's arrangement:
C -> B -> A
3. Finally, move cursor away from C's preview:
B -> A -> C
In this case, when user closes Activities, they would see window
stacking wrong for a while.
This commit adds some extra logic in _restack, checking the
_stackAbove's _stackAbove when this._stackAbove._overlayShown is true.
Though it's still not guaranteed to be always consistent as there could
be several WindowPreview with _overlayShown as true if pointer moves
really fast, this helps avoid glitches in many cases.
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4638.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3460>
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