If you maximize the window and still part is off-screen, then it’s likely a problem with your monitor configuration.
Every standalone monitor has settings that can fine tune horizontal and vertical “stretch” and shift.
You should not see the problems you describe in a laptop monitor unless your resolution is something other than whatever is default.
If you insist on something other than the default resolution, then you may have to try several until you find what works best.
I think I understand and can ‘partially’ replicate what you’re trying to convey. I can stretch (resize) a window such that it is too long to fit within my desktop, but right-clicking on the title bar, then More Actions > Resize allows me to correct that issue. Full screen should display correctly though, and I can see from the images you’ve posted that is not the case. The pertinent config file is ~/.config/systemsettingsrc. It might be interesting to see what the configuration looks like.
You could try deleting it,and then check the full-screen behaviour again.