xdg_toplevel configure with undefined window dimensions

I recently came across an issue in mutter that is related to the xdg_toplevel.configure event: https://gitlab.gnome.org/GNOME/mutter/issues/928

In the presented case, a xdg toplevel surface is created with a 200x200 pxl buffer, and additional decoration surfaces. The second xdg_toplevel.configure event from mutter contains the correct toplevel dimension (xdg_toplevel@21.configure(200, 200, array)) whereas weston provides undefined dimensions (xdg_toplevel@21.configure(0, 0, array)).

The xdg_toplevel.configure documentation states:

This may happen when the compositor needs to configure the state of the surface but doesn't have any information about any previous or expected dimension.

Form my interpretation, weston knows about the expected dimensions of the toplevel surface, since the buffer size of the attached wl_surface is known, and must provide those with the configure event.

Due to the different behaviour in weston, I would like to ask for clarity, when the toplevel configure event must include valid window dimensions (>0) and when not.