Applications stop receiving touch updates when the finger moves outside of the window

[Alessandro Astone]

[ Impact ]

A touchscreen input sequence is supposed to "grab" onto the window where the action started, so that even when moving the finger outside of the window updates are still sent to it. Instead, as the finger leaves the surface of the window, that stops receiving all touch updates and does not even receive a finish event.

[ Test Case ]

0. Log in to a GNOME Wayland desktop session
1. Open the Nautilus file manager
2. Navigate to a directory with many files
3. Switch to the vertical "List View"
4. Start scrolling vertically with the touchscreen
5. While scrolling, let the finger leave the surface of the window to its right
7. Verify that Nautilus keeps scrolling even with the finger outside the window

[ Where problems could occur ]

mutter is the Wayland compositor library that provides graphics and inputs for the GNOME desktop, thus is a critical component.
Problems could manifest with touch input not working anymore, or sudden log-out if gnome-shell crashes.
The change is limited to touchscreen handling, and won't ever affect a system without touchscreen.

[Alessandro Astone (aleasto)]

This is a regression from noble, which is unaffected.

[Alessandro Astone (aleasto)]

Fix proposed in https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4151
and then included in https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4166

[Daniel van Vugt (vanvugt)]

Comment #1 says Noble is unaffected but the proposed patch claims to fix it for Noble:
https://salsa.debian.org/gnome-team/mutter/-/merge_requests/134