`popup_unconstrain` uses view coordinates to init the output box for
popups. However wlroots expects the box to be set in a toplevel surface
coordinate system, which is not always equal to view. The difference
between those is a window geometry set via xdg-shell.
GTK4 reserves some space for client-side decoration and thus has a
window with top left corner not matching to (0, 0) of a surface. The box
calculated without taking that into account was slightly shifted
compared to the actual output and allowed to position part of the popup
off screen.
(cherry picked from commit aa443629b58e1d3d10cf64e689b661c076808d66)
1a5db66366