X11 has the notions of "transient for group", and while it's an ICCCM
violation, it's commonly used and documented that a window manager
would treat a window with transient_for set to None to transient for all
windows of its group.

gtk uses this when an application sets a dialog type window but does not
specify an explicit transient.

While this works on X11, there is no such thing as groups in Wayland and
the closest equivalent which is set_parent() in xdg-shell takes only one
parent. This is what is used for modal dialogs.

To get something similar in behavior to what is available on X11, a
solution is to update the parent() of the dialogs without transient when
the active surface changes.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=759161