When the panel is long enough to overflow the screen, it do so under wayland. That's because we don't know the absolute position of the window under wayland, so we cannot compute the right maximal height.

Downstream bug: https://bugzilla.redhat.com/show_bug.cgi?id=1780834

Since we don't know the absolute position of the window on workspace we cannot
compute the maximal height of the opened panel.

To workaround it, we'll use the height of the window as a maximal height
of the panel under Wayland.

:jhorak isn't gone, judging by bmo activity (less than 1 month ago). Bad bot.