https://bugs.kde.org/show_bug.cgi?id=477454
Bug ID: 477454
Summary: Root background damage regions are calculated
incorrectly with multiscreen
Classification: Plasma
Product: kwin
Version: 5.27.9
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: multi-screen
Assignee: kwin-bugs-n...@kde.org
Reporter: l...@asahilina.net
Target Milestone: ---
Created attachment 163418
--> https://bugs.kde.org/attachment.cgi?id=163418&action=edit
Screenshot of the issue
SUMMARY
With kwin_wayland multiscreen and no root window (no plasmashell), the black
background on screens not at the (0,0) origin is not repainted properly when
damaged.
STEPS TO REPRODUCE
1. Configure multiple screens
2. killall plasmashell
3. Move things around (or just the mouse cursor if using software cursors)
OBSERVED RESULT
See attachment. It looks like the damage/repaint region for the black
background is calculated incorrectly, and the background render happens at the
wrong output coordinates within the framebuffer. Moving the mouse cursor causes
black trails at the offset position on top of other window content (though I
couldn't grab a screenshot of this). On the screenshot you can see that only
the right band of the background is cleared to black. This is because the
repaints are offset by the width of the leftmost screen (which is less wide
than the rightmost screen).
If there are two screens with dimensions 100x100 and 200x200 placed left to
right in that order, top-aligned, then background damage at offset (0,0) in the
rightmost screen, which is at (100,0) in absolute coordinates, gets repainted
at (100,0) *relative to the rightmost screen*, which is (200,0) in absolute
coordinates. Then the region at (100,0) is missing the background repaint, so
whatever damaged that region just paints on top of the stray/damaged
framebuffer (leading to cursor trails and the like), while the region at
(200,0) gets clobbered with black even if there is some other window on top.
EXPECTED RESULT
Correct rendering
SOFTWARE/OS VERSIONS
Operating System: Fedora Linux Asahi Remix 39
KDE Plasma Version: 5.27.9
KDE Frameworks Version: 5.111.0
Qt Version: 5.15.11
Kernel Version: 6.6.0-asahi-00861-g9b58b16aa50d (64-bit)
Graphics Platform: Wayland
Processors: 12
Memory: 54.5 GiB of RAM
Graphics Processor: Apple M2 Max
Product Name: Apple MacBook Pro (16-inch, M2 Max, 2023)
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