Inadvertent full-screen in editor

[vr8ce]

7.16 on MacOS Sonoma 14.4.1.

Longtime user of calibre, but I've never used it (or anything else) fullscreen. While editing a book today, I hit some unknown key that sent the Edit Book window to fullscreen. I could find no button on the Edit toolbar to control it, nor did I see anything in the Edit Book's preferences, nor do I see any entry on the Edit Book menu, nor did I see anything in Keyboard shortcuts. I could only get it out of fullscreen by opening the Mac spaces and toggling the fullscreen icon.

But, any new book I opened also went to full-screen. I managed, by accident, to semi-fix that—I resized one book, and subsequent opens of any book now resize to that size (the "Restore state of previously edited book when opening it again" option is unchecked). However, now when I edit any book, it opens fullscreen and then immediately resizes itself, so you can see the flash. I tried resetting the "Edit Book" preference defaults, and tried reinstalling calibre, neither of which had any effect.

1. What is the calibre way to get the Edit book window in and out of fullscreen?
2. How do I stop calibre from initially opening an Edit book window fullscreen?