This was the previous behavior for both the top and bottom toolbars.
Regressed when changing to use a new custom behavior for the top toolbar.

When entering fullscreen we will now collapse and hide the toolbar, allow the
browser to expand to the entire screen estate and then, when exiting fullscreen
expand the toolbar.
Collapsing and then expanding the toolbar will trigger the
EngineViewBrowserToolbarBehavior to place the browser below the toolbar.