Debug mode MUTTER_DEBUG_TRIPLE_BUFFERING=always doesn't work anymore

[Daniel van Vugt]

[ Impact ]

Debug mode MUTTER_DEBUG_TRIPLE_BUFFERING=always doesn't result in triple buffering being used anymore.

[ Test Plan ]

1. Add MUTTER_DEBUG_TRIPLE_BUFFERING=always to /etc/environment
2. Reboot.
3. Log into a Wayland session.
4. Open a Terminal window and drag it in circles with the mouse.
5. Verify there is a slight lag between the mouse pointer position and the window position. This means it's always triple buffering (or you accidentally logged into a Xorg session).

[ Where problems could occur ]

Anywhere in frame scheduling, including screen freezes and stutter.

[ Other Info ]

This regression occurred in 46.0 as a result of jbicha and myself working a little too independently. Jeremy identified that the upstream branch supported 46 and cherry-picked that for the Noble release, while I was still working on creating a stable maintenance branch for 46 in https://gitlab.gnome.org/Community/Ubuntu/mutter/-/tree/triple-buffering-v4-46

Fortunately it's very low priority and nobody has actually reported it.

[Daniel van Vugt (vanvugt)]

According to my upstream notes it could have caused a freeze, but that's not what I am seeing in Noble. Just regular double buffering.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1441#note_2099140

[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 plan in the bug description.