I absolutely can't stand Full Screen mode. Never, ever use it and don't want to. This is not about that "full screen".
I have always made my mail client window full screen size by either double-clicking the title bar, or since I use BetterTouchTool, just dragging it to the top of the screen and it instantly goes full size. Simple.
Today, this doesn't work any more. The mail window will never go full screen, only to some pre-determined maximum size that leaves considerable space at the sides and bottom. No amount of dragging will go larger, and double-click on the title bar maxes out at this new, sub-full-screen size.
Works normally in all other apps, double-click title bar and they take up the entire window.
What gives here? This is a new behavior, just started this morning. Anyone have any idea why this is happening and how to stop it? macOS Catalina.
EDIT: Today, it's not doing this any more. I have no idea why. I had even rebooted yesterday morning and it was still doing it. But not today. Very strange!