Fullscreen toggle by window manager does not change OS.window_fullscreen property

[Rubonnek]

Godot version:
3.2 branch - 63fd0d3

OS/device including version:
Arch Linux

Issue description:
Toggling fullscreen mode on and off using i3 as the window manager does not update the value of OS.window_fullscreen.

I suppose this may be happening with other tiling/dynamic window managers.

Steps to reproduce:

1. Launch any project in non-fullscreen mode.
2. Toggle fullscreen using the window manager shortcut (usually Alt+F in i3)
3. Check OS.is_window_fullscreen(). It returns False.

Minimal reproduction project:

fullscreen_toggle.zip

[Calinou]

This is probably expected. Can the application even be aware of this? Is there any precedent of an application noticing when you use the window manager to make it fullscreen or borderless?

[Rubonnek]

I believe it can be made to be aware of it. Firefox can toggle its full screen mode successfully by using its shortcut F11 and the window manager shortcut Alt+F interchangeably in any order under X11. I'm not sure how Firefox does it yet.

I haven't tested anything on wayland, but I suppose the same applies.

[rsubtil]

Looking at the FreeDesktop specification for _NET_WM_STATE, and in this sentence in particular:

A Client wishing to change the state of a window MUST send a _NET_WM_STATE client message to the root window (see below). The Window Manager MUST keep this property updated to reflect the current state of the window.

leads me to believe that the window manager should be sending _NET_WM_STATE_FULLSCREEN atoms to Godot

[akien-mga]

I can confirm the issue on KDE / KWin too when setting Fullscreen with Right click on taskbar > More > Fullscreen.

[akien-mga]

Fixed by #62543.

[akien-mga]

Still needs to be fixed in 4.0 though (I checked and it's reproducible), so let's keep this open for now.