Help: I fullscreened a window and can't undo it

I fullscreened the "Picture in Picture" window in Vivaldi browser and now I can't undo it. The window will disappear if the taskbar is visible, and older posts I've googled up suggesting ctrl-alt-F and ctrl-shift-F don't help. The Window Rules category of Settings is empty.

Is there maybe a file I can edit to remove this rule or something? I'm out of ideas.