A custom toplevel window derived from GtkWindow can have non-zero x/y
allocations, whereas for a Gtk toplevel window they are usually zero.

Assuming the latter, the translation of the coordinates for tooltip
positioning is relative to 0/0, resulting in incorrect positioning for
custom windows such as in https://bugs.debian.org/1099113.

The allocation of a custom toplevel window is relative to the screen origin,
so the position of the child must also be relative to the screen origin in
order to get the correct coordinates relative to each other.

This bug has been reported upstream, including the attached patch
(https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/8216).