AHA!! I lied in my first post. A custom column
was the culprit. After deleting my gui.json file and letting calibre rebuild it, everything was fine. Changed to calibre's interface style ...
still fine. Changed the icon size on the toolbars to small ...
still fine. Removed the stuff from the toolbars that I always remove so that everything fits on my tiny screen ...
still fine. Changed libraries to one that had a custom Read/Unread column ... BOOM! Quarter-sized visible main window that the mouse believed was filling the screen.
I don't need the Read/Unread column that much (I forgot I set it up, to tell the truth), so that's my fix for now. Maybe the info will help someone else.
Thanks Manichean! Your nudge resulted in me nailing down my issue.
EDIT: GUI debug mode had absolutely no relevance to my situation. The fact that I had changed libraries to one without a custom column is the reason I had it working all along.