https://bugs.kde.org/show_bug.cgi?id=452494
Bug ID: 452494
Summary: gtk/gdk applications are locked to 60 fps when using
kwin as an x11 compositor
Product: kwin
Version: git master
Platform: unspecified
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: compositing
Assignee: kwin-bugs-n...@kde.org
Reporter: marcus.gur...@gmail.com
Target Milestone: ---
SUMMARY
***
gtk/gdk based applications are locked to 60 fps when using kwin_x11.
The cause for this is gdk requiring the "_NET_WM_FRAME_DRAWN" protocol which is
not implemented in kwin. Since the protocol is not advertised by kwin, gdk
falls back to a fixed 60Hz frame clock.
***
STEPS TO REPRODUCE
1. Use kwin_x11 on a display with >60Hz refresh rate.
2. Open any gtk application, e.g. Xournal++.
OBSERVED RESULT
The application does not synchronize to vblank and instead updates at a fixed
refresh rate of 60Hz. In Xournal++ this manifests in much increased input
latency and reduced smoothness.
EXPECTED RESULT
The application synchronizes to vblank. This is the behaviour found with
mutter.
SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Manjaro
(available in About System)
KDE Plasma Version: 5.24.4
KDE Frameworks Version: 5.92.0
Qt Version: 5.15.3
ADDITIONAL INFORMATION
The protocol seems to be non standard since it is not listed in
https://specifications.freedesktop.org/wm-spec/latest/ar01s06.html
_NET_WM_FRAME_DRAWN is described here:
https://fishsoup.net/misc/wm-spec-synchronization.html
Here gdk checks whether the protocol is supported:
https://github.com/GNOME/gtk/blob/53013676309522d08e4f30257b7bfd17d68bead2/gdk/x11/gdksurface-x11.c#L593
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