Bug 445611

Created attachment 143641 [details]
RetroArch screenshot

SUMMARY
When running a fullscreen application that uses OpenGL (primary use case are video games) with desktop effects disabled (the application disables them or manually with Shift+Alt+F12, Spectacle often produces pixel garbage in the screenshots.

Attached are two screenshots showing the issue. One is from RetroArch, the other with plain old glxgears after switching it to fullscreen mode using Alt+F3/More Actions/Fullscreen.

STEPS TO REPRODUCE
1. Disable the compositor (default shortcut Shift+Alt+F12.)
2. Run glxgears.
3. Make it fullscreen through KWin (upper-left icon on the window title bar, "More Actions", "Fullscreen".
4. Press Shift+PrintScr multiple times to take several screenshots and automatically save them in the default screenshot location configured in Spectacle.

OBSERVED RESULT
Some of the screenshots will have thick horizontal bars of black pixels.

EXPECTED RESULT
Screenshots should be clean without black pixels.

SOFTWARE/OS VERSIONS
Operating System: Gentoo Linux 2.8
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Kernel Version: 5.10.79-gentoo (64-bit)
Graphics Platform: X11
Processors: 16 × AMD Ryzen 7 3700X 8-Core Processor
Memory: 15.6 GiB of RAM
Graphics Processor: NVIDIA GeForce GTX 980 Ti/PCIe/SSE2

ADDITIONAL INFORMATION
I installed flameshot (a screenshot tool, https://github.com/flameshot-org/flameshot) and it does not have this issue.