Created attachment 136081 [details]
OpenGL Rendering fault with Scantailor - I can provide examples from at least 3 more apps if necessary

SUMMARY
I am experiencing numerous issues related to Qt-based software with OpenGL(ES)-accelerated graphical interfaces.
The part of the application that should be drawn by Qt OpenGL(ES) is either completely invisible (on Wayland, even the application or wallpaper behind is shown), and/or severely suffering from rendering artifacts / flickering.
Some affected/tested applications:
- Gwenview (activate OpenGL in settings) [neon package]
- OBS Studio [neon package]
- ScanTailor Advanced (activate OpenGL in settings) [self-built]
- Olive Video Editor 0.1 branch [ubuntu package]
- Olive Video Editor 0.2 branch [self-built]

The enormous similarity of the issues, affecting both official builds from neon and self-built applications, makes me think that this problem may likely be caused by malconfigured Qt OpenGL(ES) packages in KDE Neon. It is rather unplausible that these problems are caused by my setup or configuration since all non-Qt OpenGL-related stuff works well.

STEPS TO REPRODUCE
1. Install or build one of the applications mentioned above (or, potentially, any application depending on libqt5gui5-gles or libqt5opengl5)
2. Open the respective application and (if necessary) activate OpenGL (there might be a need to restart)
3. Open/load an image, video, or whatever

OBSERVED RESULT
Applications using Qt OpenGL modules do not render footage/media at all, or only with extreme flickering and rendering artifacts.
-> On Wayland, you will see the application behind (or the desktop wallpaper) in the frame where the footage should be rendered
-> On X11, you will see nothing (only the background colour)

EXPECTED RESULT
Content should render at all on Qt-OpenGL(ES), and without artifacts or flickering. The stuff behind should not be visible in the content frame.

SOFTWARE/OS VERSIONS
Operating System: KDE neon 5.21
KDE Plasma Version: 5.21.0
KDE Frameworks Version: 5.79.0
Qt Version: 5.15.2
Linux Kernel Version: 5.4.0-66-generic
OS Type: 64-bit
Graphics Platform: Wayland (X11 is affected as well)
Processors: 4 × Intel® Core™ i5-3570K CPU @ 3.40GHz
Memory: 7.2 GiB of RAM
Graphics Processor: Mesa DRI Intel® HD Graphics 4000

ADDITIONAL INFORMATION
- The issue already existed in previous versions of neon (5.20)
- A possible idea is that a partial cause might be the libqt5gui5-gles package, which is not installable as it's still the Ubuntu version (but this cannot be the only reason, since Gwenview does not depend on this package, but is affected too)
- I am not certain whether this is related or not, but my self-built OpenToonz (latest stable release tarball) crashes on startup with a segmentation fault because the EGL context could not be created (only on Wayland, though; it at least comes up on X11)

DISCLAIMER
Although I believe these problems might be caused by neon packaging mistakes, I am not sure; it's nothing but an assumption. I might easily be wrong (it could even be an issue with Qt, for instance, or something hardware specific). However, I chose to report this here because the distribution seems like the first place to ask for help.

NOTES ON REPRODUCIBILITY
Information on whether other users can reproduce this issue or not would be important. Reproducibility might also depend on the hardware / drivers used.