Fullscreen direct scanout is not used if you enable the hidden experimental VRR feature in Mutter 46

[Daniel van Vugt]

[ Impact ]

Fullscreen direct scanout is not used if you enable the hidden experimental VRR feature in Mutter 46.

[ Test Plan ]

0. Find a monitor that is VRR capable.
1. Set CLUTTER_PAINT=damage-region in /etc/environment
2. gsettings set org.gnome.mutter experimental-features '["variable-refresh-rate"]'
3. Reboot.
4. Log into a Wayland session.
5. Don't panic. The screen is meant to turn red whenever something changes.
6. In Settings > Display verify that variable refresh rate is detected and enabled.
7. Open Google Chrome (Firefox won't work) and start an animation like https://www.vsynctester.com/
8. Press F11 to make it full screen.
9. Verify the screen has now stopped being red.
10. Press F11 again to exit full screen.
11. Verify the screen has gone red again.

If the screen went black at the end, don't panic. That's a separate bug 2066080.

[ Where problems could occur ]

Since the fix affects the runtime decision of whether to allow triple buffering or stay double buffering, the main risk is that triple buffering wouldn't be used in some situation where we would rather have it to maintain a smoother frame rate.

[ Other Info ]

First reported upstream in:
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1441#note_2062222

We DO NOT recommend enabling the VRR feature anyway because it is still buggy (bug 2066080)

[Timo Aaltonen (tjaalton)]

Hello Daniel, or anyone else affected,

Accepted mutter into noble-proposed. The package will build now and be available at https://launchpad.net/ubuntu/+source/mutter/46.2-1ubuntu0.24.04.1 in a few hours, and then in the -proposed repository.

Please help us by testing this new package. See https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how to enable and use -proposed. Your feedback will aid us getting this update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug, mentioning the version of the package you tested, what testing has been performed on the package and change the tag from verification-needed-noble to verification-done-noble. If it does not fix the bug for you, please add a comment stating that, and change the tag to verification-failed-noble. In either case, without details of your testing we will not be able to proceed.

Further information regarding the verification process can be found at https://wiki.ubuntu.com/QATeam/PerformingSRUVerification . Thank you in advance for helping!

N.B. The updated package will be released to -updates after the bug(s) fixed by this package have been verified and the package has been in -proposed for a minimum of 7 days.

[Daniel van Vugt (vanvugt)]

Verified fixed in 46.2-1ubuntu0.24.04.1 using the test case in the Bug Description.