Commit 1ea22a5281c broke the window reposition animation when it
based the ::size-changed signal on the layout manager's bounding box
instead of the MetaWindow::size-changed signal.
That's happening because of the combination of:
1. we adjust to window size changes immediately without animations
2. closing a window triggers a change to a 0x0 bounding box which
is not treated as a size change
Fix this by addressing the 2nd factor, and don't treat a change to
a 0x0 bounding box as size change.
https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2901
b2c35e4fb0