It was quite difficult to look up for Windows, and X11 has its own more complicated way of doing things. On X11, I’ve managed to enter fullscreen, but exiting is even more difficult and less documented.
I know Wayland exists, but as long as XWayland provides a good enough support, I’ll use that instead for now, due to lack of time on my own side.
EDIT: To clarify: I meant how do I do it via API calls, in programming, not by what key is the default (which is F11).
If you mean at the X11 call level, I think that it’s a window hint, assuming that you’re talking about a borderless fullscreen window, and not true fullscreen (like, DGA or DGA2 or something, in which case you don’t have a fullscreen X11 window, but rather direct access to video memory).
https://specifications.freedesktop.org/wm-spec/latest/ar01s05.html
See
_NET_WM_STATE_FULLSCREEN, ATOM
If you’re using a widget toolkit like gtk or something and writing the program, it’ll probably have some higher-level fullscreen toggle function that’ll flip that on X11. Ditto for SDL.
If you mean in a script or something, I’d maybe try looking at
xprop(1)
to set that hint.I’d also add, on the “user” front, that I don’t use F11 and I think that that every window manager or desktop environment that I’ve ever used provides some way to set a user-specified keystroke to toggle a window’s fullscreen state. I’ve set Windows-Enter to do that for decades, on every environment I’ve used.