which fills the toplevel window, rather than directly to the toplevel window.

It makes no difference for X, but for wayland the window decorations are part
of the toplevel window, dropping down a level means we don't draw out menu bar
under the window decoration space

Change-Id: Icec400efacd16b5d901107c13b6fa90c59cad0e6
(cherry picked from commit 298c089d)