Created attachment 139815 [details]
Vertically "maximized" Kate window with 1 pixel gap at bottom.

SUMMARY

Using X11, if a window or application is set to vertically maximize, when reopened it will have a 1 pixel vertical gap between it an the edges of the screen (i.e., its vertical height will be 1 pixel less than it should be).

STEPS TO REPRODUCE
1. Set Global Scale to >175% or above.
2. Open a window that should be vertically maximized (but not fully horizontally).

OBSERVED RESULT
Window will not be fully vertically maximized, and a gap of 1 pixel will be visible.

EXPECTED RESULT
No gap should be visible.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 21.04
(available in About System)
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
This is obvious when a window is tiled to one size of the screen, then reopened. It also occurs even if a window rule is set to make the window maximized vertically initially, thus ignoring that setting. If a rule forces the window to be that size at all times, it does obey the rule (but then, of course, the user can't manually resize it to anything else).

Sometimes the gap appears at the top, sometimes the bottom. The gap appears when using a plasma panel, a latte panel, or no panel at all.

This does not occur if a window is fully maximized (those will reopen fully maximized both vertically and horizontally) nor windows just set to be maximized horizontally.

In my testing it seems that this only happens when the global scale in the display settings is set to 175% or more (no gap appears at 100% to 150%).