There have been several bugs in the past that caused invalid transitions
of the `shown` state, such as going from `showing` to `showing`. These
cause consecutive emissions of the `showing` signal, which can confuse
other classes such as the search controller which connects to the stage
`key-press-event` on showing and disconnects again on `hiding`. Having
two consecutive `showing` signals will cause it to connect twice, and
only disconnect once when hiding the overview again. This will lead to
key presses getting repeated in the search until the session is
restarted. Because there is no obvious connection to how and when this
issue got triggered, this now adds some validation code that only allows
valid transitions and throws an error otherwise so we get a backtrace of
the code actually causing the problem.
This does not fix the issue(s) causing the invalid state transitions, it
only adds a way of tracking them down.
Related: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4651
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2514>
a91b6439ba