Why does Command-` on macOS stop working when an app is fullscreen?

@CraigStuntz the whole concept of Spaces on macOS drives me nuts.

Just because I want a window to be fullscreen doesn't mean i want an entirely new and separate virtual desktop space!!!

@jasonkarns @CraigStuntz pro tip: hover or click-and-hold on the green window button, select the single large rectangle in "Fill & Arrange", et voila a screen-sized window that is not in full-screen mode

You also might like Settings > Desktop & Dock > Window title bar double-click action > Fill

@gparker I'm trying to watch a video. I full-screen the embedded video player. Command-Tab then works, but Command-` does not.