[elygre]

Even with a single screen I sometimes want a video in full screen, while keeping an app window (email, excel, whatever) in a window in front of it.

[alex_duf]

right click -> "always on top". That's available by default

[mort96]

Well, that only works for GTK 3 or 4 apps. Thanks to CSDs and GNOME's refusal to implement SSDs, every app that's not either running in XWayland or a GTK3/4 app has its own decorations without the context menu with the always on top option.

There was some way of enabling always on top on non-GTK3/4 apps too, but I don't remember it off the top of my head.