https://bugs.kde.org/show_bug.cgi?id=414339
Bug ID: 414339
Summary: Some Kwin vsync settings produce severe screen
flickering
Product: kwin
Version: 5.14.5
Platform: Debian testing
OS: Linux
Status: REPORTED
Severity: major
Priority: NOR
Component: compositing
Assignee: kwin-bugs-n...@kde.org
Reporter: fancyname+kdebugtrack...@use.startmail.com
Target Milestone: ---
Created attachment 124032
--> https://bugs.kde.org/attachment.cgi?id=124032&action=edit
Kwin support information while vsync is set to "Full screen repaints" is
enabled
SUMMARY
After my system updated KWin to 4:5.14.5-1+b1 (debian bullseye) and rebooted,
the screen heavily starts flickering as soon as kde is loading. After some
testing, I could track down the issue to kwin and especially to the Tearing
prevention ("vsync") setting.
When "Enable compositor on startup" is deactivated and I logout and login
again, everything is fine. In this case, I only noticed some flickering when
typing in the password on the lock screen and sometimes in kde panels.
STEPS TO REPRODUCE
1. Go to settings, Display and Monitor, Compositor
2. Set "Enable compositor on startup"
3. Set "Tearing prevention ("vsync")" to anything else than "Never"
4. Apply and restart kwin
OBSERVED RESULT
Here are some more details of the behaviour with different kwin settings:
When using OpenGL2.0 and 3.1:
Flickering occurs with both versions, I didn't recognize any difference.
Behaviour under different vsync options:
- Never: Pretty stable, only the kde panel is still buggy. When hovering
over the panel, it flickers and soon after freezes partly. Widgets on it don't
update their appearence (e.g. clock stops) but still work when clicking on
them.
- Automatic / Only when cheap: Flickers horribly. Seems that windows are
z-fighting. When moving windows, they are still displayed on the same position
while their actual position is moved correctly. System is totally unusable.
- Full screen repaints / Re-use screen contents: Flickers over the whole
screen when moving something (mouse included) but the display is still updated
correctly. One can still interact with the system although it is cumbersome.
Using XRender:
There is only flickering on KDE startup, afterwards it behaves normal with all
vsync options.
One exception is hovering over an kde panel. In that case, the panel flickers
while all other elements remain stable.
There are a lot of related bugs, e.g. this one:
https://bugs.kde.org/show_bug.cgi?id=383179
If you need anything else, please tell me.
EXPECTED RESULT
No flickering.
SOFTWARE/OS VERSIONS
Linux: Debian testing (Bullseye), kernel 5.2.0-3-amd64
(available in About System)
KDE Plasma Version: 5.14.5
KDE Frameworks Version: 5.62.0
Qt Version: 5.12.5
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