Window previews can spill out of the container when their position in the
session overflows the monitor boundaries. In the past, Workspace didn't have
a visible background, and was (almost) always fullscreen, which would mask
this particular problem. However, nowadays, it is very much noticeable when
this situation happens.
Clip the window previews container to its allocation when the overview state
is bigger than WINDOW_PICKER. That is, between HIDDEN and WINDOW_PICKER states,
inclusive, no clipping is applied.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1691>
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