GTK4: Bar window appears over fullscreen window if started after the window

[kfstorm]

Description

In AGS v3 (GTK4/Astal), if a window with Astal.Exclusivity.EXCLUSIVE (e.g., a bar) is created while a fullscreen application is already running on the same monitor, the bar appears on top of the fullscreen application.

The expected behavior is that the bar should respect the fullscreen state and be hidden behind it or not overlap, as it usually does if started before the fullscreen app.

Steps to Reproduce

1. Start a fullscreen application (e.g., a game or browser in F11).
2. Start AGS with a bar configured using:

<window
  visible
  exclusivity={Astal.Exclusivity.EXCLUSIVE}
  anchor={Astal.WindowAnchor.TOP | Astal.WindowAnchor.LEFT | Astal.WindowAnchor.RIGHT}
  application={app}>
  ...
</window>

3. The bar is rendered on top of the fullscreen app.

Environment

• AGS version: 3 (Astal/GTK4)
• Desktop Environment: Hyprland (Wayland)
• OS: Linux