clutter_stage_schedule_update() sets the field `update_scheduled` to
`TRUE` as an optimization to make redundant updates a no-op. This failed
if there was a pending event and if the stage was not yet mapped.
What happened is:
* clutter_stage_schedule_update() is called
- ClutterStage::update_scheduled is set to TRUE
- frame clock scheduled
* frame clock dispatches
- frame is discarded early, no actual stage update happens
* device is created (e.g. virtual device from remote desktop session)
- `device-added` event reaches ClutterStage::event_queue
* stage is shown
- clutter_stage_schedule_update() is called
- ClutterStage::update_scheduled is TRUE
- ClutterStage::event_queue has events in it
- These two conditions means clutter_schedule_update() becomes a
no-op
At this point, no more updates will happen from
clutter_stage_schedule_update().
Fix this by resetting `ClutterStage::update_scheduled` to `FALSE` even
if the frame was discarded due to the stage not yet being mapped.
A test case is added that replicates the above descibed events.
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/3804
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4152>
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