Find a better way to detect fullscreen changes

[tsipinakis]

References: #613, bbidulock/icewm#347

As mentioned in the issues above we're currently relying on the window manager to send either a PropertyNotify or ConfigureNotify on the root window in order to update the fullscreen status, to hide and show notifications as appropriate.

However, this isn't directly EWMH compliant. And unfortunately from my research there is no simple way to do this with EWMH hints.

The one way I can think of would be to monitor the Focus{In,Out} events in order to subscribe to PropertyNotify events of each window when focus is gained and unsubscribe when focus is lost. We'd then have to monitor the _NET_WM_STATE_FULLSCREEN hint on that window.

Unfortunately this seems quite cumbersome to implement on a first look. But unless there's another solution this seems the only way to be properly compliant.

[bynect]

@fwsmit do you know if any work was done into this regard?

[fwsmit]

Not that I remember. On wayland this is not an issue, though