SUMMARY
When selecting "Mirror Display" on Wayland(possibly on X11, I have not tested it), the displays will be aligned through the top of the display rather than the center. This provides for a displeasing user experience(for example, watching 16:9 content on this setup through MPV will cut off the bottom of the video but have a black bar on top) when using 16:9 external displays with 16:10 laptops, which is a very common configuration today.

STEPS TO REPRODUCE
1. Have device with 16:10 or other native display
2. Connect 16:9 external display or other
3. Mirror Displays
4. Open a program where the bottom 100 or so pixels are important for the user experience

OBSERVED RESULT
Content will be cut off on one of the displays

EXPECTED RESULT
The compositor should align the aspect ratios of the displays, with the user selecting which display's aspect ratio should be preferred(iirc this is how Apple does it, and it provides a very good experience when I used it)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 5.22.3
(available in About System)
KDE Plasma Version: 5.22.3
KDE Frameworks Version: 5.84
Qt Version: 5.15.2

ADDITIONAL INFORMATION