On x11 we emulate pointer events from touch events as long as there's
only one touchpoint on screen, this obviously leads to x11 sending us
crossing events triggered by the emulated pointer. Now if we get a leave
event and set the stage of the ClutterInputDevice to NULL, new touch
events will be discarded by clutters backend because the core pointer
doesn't have a stage associated. This means Mutter completely loses
state of a touchpoint as soon as it crosses a shell actor.
An easy reproducer for this issue is to start the four-finger-workspace
gesture above a window and to move the pointer emulating touch outside
of the window, this will freeze the gesture as the gesture no longer
receives touch events.
To fix this, stop tracking stage changes on crossing events and simply
leave the ClutterInputDevice stage as-is. In our case there is only one
stage anyway and that won't change in the future.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/423
b98ebe60b2