I just realized that notifications appear on top of fullscreen windows.
I will make xnotify detect when the active window is fullscreen and apply one of the following behaviors.

For people who use other notification systems, what happens when you have a window in fullscreen?
• Does the notification is ignored and never appears?
• Does the notification is queued and appears when the fullscreen window become normal-sized or is closed?
• Or does the notification appears on top of the fullscreen window anyways?

Which behavior do you prefer in a notification system?