When starting a gesture to open the overview while a transition to hide
the overview is running, Overview._shown will be first set to false when
starting the transition and then to true by the gesture before the
`onComplete` callback is called. The `onComplete` callback in this case
is `Overview._hideDone()` which starts a transition to show the overview
again which also emits the `showing` signal. Since the gesture emits a
`showing` signal as well, this results in two consecutive `showing`
signals without a `hiding` signal in between.
This breaks the `searchControler` which adds a key press handler to
start the search on `showing` and removes it on `hiding`. So every time
this happens a new handler that will never be removed is added,
resulting in the first key press being repeated.
Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4004
Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/3819
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1828>
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