I just made a few qss edits to Shellnut’s DOS Terminal theme.
Mellow Yellow does indeed have the same bug as dark mode and DOS Terminal.
Interestingly, I tried light mode again and I can get the bug to appear by playing with Windows’ “Automatically hide the taskbar” setting. But if I click OK when the Options window is the correct size, it will remember that size. But this detail doesn’t really matter because no one will toggle “Automatically hide the taskbar” anywhere near as much as I have while exploring this bug.
The core difference seems to be that in light mode, you can vertically shrink the Options window significantly more than you can in other themes. The question is why.