Bug 445628

SUMMARY
First of all, sorry if it's not the right product or component to report the bug. Please, move it where it fits better, if you know better.

Today I discover that if I change the resolution from 1920x1200 (16:10) to 1920x1080 (16:9), when I change it back to the 16:10 one, there is a black region on the bottom. It is not just a render issue, because the mouse cannot move to it. It's hard to explain, but I hope you get the idea. Maybe I can add a video if it's not clear.

I realized this by connecting my laptop to an external TV with HDMI, unifying output (then resolution changed automatically to a 16:9), and then removing the HDMI. First thing I noticed, is that laptop didn't revert to its native 16:10 resolution (should I open a different report for this?), which is a thing I would expect, but then I saw the main bug that I described above when I tried to select the right resolution manually.

In any case, I can reproduce the main issue without attaching the laptop to an external monitor.

STEPS TO REPRODUCE
(I guess a laptop with a 16:10 aspect ratio screen is needed)
1. Go to System Settings -> Display and Monitor -> Display Configuration, select 1920x1980 (16:9) and apply.
2. Change back to 1920x1200 (16:10) and apply

OBSERVED RESULT
There is a black region on the bottom of the screen that shows nothing. Mouse also cannot pass to that region.

EXPECTED RESULT
Resolution changes successfully.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.88.0
Qt Version: 5.15.2
Kernel Version: 5.15.2-arch1-1 (64-bit)
Graphics Platform: X11
Processors: 8 × 11th Gen Intel® Core™ i5-1135G7 @ 2.40GHz
Memory: 15.4 GiB of RAM
Graphics Processor: Mesa Intel® Xe Graphics