Xorg sessions have higher output lag than Wayland sessions

[Daniel van Vugt]

Upstream bug: https://gitlab.gnome.org/GNOME/mutter/issues/334
Upstream fix: https://gitlab.gnome.org/GNOME/mutter/merge_requests/281

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Xorg sessions have higher output lag than Wayland sessions.

Test case:
Grab a window with the mouse and drag it around smoothly.

Expected: the same minimal gap between the cursor and the window's title bar in both Xorg and Wayland sessions.
Observed: a bigger gap (higher latency) in Xorg sessions.

I only realized this was a bug when I stumbled on a fix while experimenting with the presentation timing logic in mutter, so assigning to myself.

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: mutter 3.30.0-4
ProcVersionSignature: Ubuntu 4.18.0-8.9-generic 4.18.7
Uname: Linux 4.18.0-8-generic x86_64
ApportVersion: 2.20.10-0ubuntu11
Architecture: amd64
Date: Sun Oct 7 17:22:48 2018
InstallationDate: Installed on 2018-05-26 (133 days ago)
InstallationMedia: Ubuntu 18.10 "Cosmic Cuttlefish" - Alpha amd64 (20180525)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_AU.UTF-8
 SHELL=/bin/bash
SourcePackage: mutter
UpgradeStatus: No upgrade log present (probably fresh install)

[Daniel van Vugt (vanvugt)]

Upstream: https://gitlab.gnome.org/GNOME/mutter/issues/334

[Daniel van Vugt (vanvugt)]

Please don't test for this bug while bug 1818972 is open. Or at least use the workaround mentioned in bug 1818972.

[Daniel van Vugt (vanvugt)]

Fix released in mutter 3.34