[Spectacle] [Bug 524305] Window Under Cursor captures Plasma desktop as fully transparent with fractional scaling on Wayland

https://bugs.kde.org/show_bug.cgi?id=524305

--- Comment #1 from Simon Zevnik <[email protected]> ---
I did some additional debugging on Fedora 44 / Plasma 6.7.4 / Wayland with
fractional scaling at 1.2.

A few useful findings:

queryWindowInfo() shows that when clicking the empty Plasma desktop, KWin
selects the same plasmashell Desktop window at scale 1.0 and 1.2. The UUID,
PID, window type and layer are identical; only the logical geometry changes
from 2560×1440 to 2133×1200.
A direct call to org.kde.KWin.ScreenShot2.CaptureInteractive(kind=Window) at
scale 1.2 can successfully capture the same desktop as a fully opaque 2560×1440
image. This suggests fractional scaling alone is not sufficient to trigger the
bug.
I also reproduced Spectacle's exact pipe-reading path (QImage + DPR +
QDataStream::readRawData). When KWin returns a correct buffer, Spectacle
receives all expected bytes and the image remains fully opaque. I have not
found evidence that Spectacle corrupts a correct buffer after receiving it.
The problem appears to be state/timing dependent. Instrumentation often makes
the bug disappear, which suggests a possible race/lifecycle issue around window
selection or capture.
In one separate transparent capture, KWin itself returned a completely
transparent raw ARGB buffer, so at least in that case the transparency was
already present before PNG encoding. However, that capture was not the exact
CaptureInteractive(Window) reproduction path, so I do not consider it proof of
the root cause of this bug.

I also built KWin 6.7.4 locally and created an integration test for desktop
screenshots at scale 1.0 and 1.2. The isolated test does not reproduce the
transparent result, including with a Plasma layer-shell desktop and dmabuf
buffer.

I can provide the test code, raw buffers, metadata and D-Bus logs if they would
be useful.

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