Created attachment 167466 [details]
Firefox showing button at viewport bottom right (before disabling ext. monitor / after applying workaround)

SUMMARY
After enabling ("Extend to right") and again disabling an external monitor, applications get the wrong impression what the current vertical viewport size actually is, although the screen resolution seems right. For example, a maximized Firefox window displaying a website with a button at the "bottom" of the current viewport does not show the button in the right place anymore, but at an even lower position behind the Plasma taskbar (see attached screenshots). Similarly, a maximized Konsole terminal shows its bottom-most text line behind the Plasma taskbar.

As a workaround, switching screen resolutions back and forth fixes the problem temporarily, until again an external monitor is involved:

xrandr --screen 0 --size 1680x1050
sleep 1
xrandr --screen 0 --size 1920x1080

STEPS TO REPRODUCE
1. Attach external monitor via HDMI that has a higher resolution (here: 2560x2880) than the primary monitor (here: 1920x1080)
2. Enable external monitor, "Extend to right"
3. Start a Konsole terminal, maximize on primary monitor, repeatedly press ENTER to reach the bottom terminal line
4. Disable external monitor, "Switch to laptop screen"

OBSERVED RESULT
Bottom terminal line (and cursor) isn't visible anymore but "behind" the Plasma taskbar

EXPECTED RESULT
Bottom terminal line (and cursor) should be visible right above the Plasma taskbar

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.27.10 (Debian testing: 4:5.27.10-3)
KDE Frameworks Version: 5.107.0
Qt Version: 5.15.10