Leave fullscreen when dragging or double-clicking headerbar
When developing Loupe I noticed that I would intuitively try to drag or double-click the headerbar to get out of fullscreen.
The visual difference between a full-screened and a maximized window is often not that big. So I would suggest that the things that work for un-maximizing a window should also work for un-fullscreening.
Afaik GTK 4 is just passing through the necessary signals. It even shows the drag cursor when trying to drag the headerbar in fullscreen.