I think I may have found an answer, buried deep in another part of this forum, in a YouTube video (probably not intentionally):
https://youtu.be/T8-vhl1tWBg1a) Download and install Visual Studio Code or Notepad++ (or similar)
1b) Download and install 7-Zip
2) Navigate to the screenset enclosing folder (ex: C:\Mach4Hobby\Screens\)
3) Double-click on a screenset
4) Right-click on "screen.xml", and select "Open Outside" (or shift + enter)
5) Change the value in line 6 to 1920
6) Change the value in line 7 to 1064
I measured my Dell U4320Q screen to have a 8mm menu bar at the top out of a total of 532mm of vertical space (in fullscreen with the taskbar hidden), so I came up with 1064 pixels instead of 1080, after rounding (ultimately this will be used on a 1080p touchscreen). Now the canvas is in a 16:9-ish ratio in the screen edit environment, and when stretched, square buttons are still square!
It wouldn't be hard to edit the entire screenset in the code editor - it might even be easier if you know how to code
Is there a way to completely eliminate the menu bar when operating the machine?