WinRT: Combobox menus are resized when re-opened

After closing and reopening a combobox menu, it is no longer fullscreen. While fullscreen menus are not ideal, they are "solution" we use on this platform for widgets. If the menu reopens as non-fullscreen, it will paint in a different place than the windowing system expects it to be.

Possible solutions:

• Check the window geometry is not being allowed to change to non-fullscreen, and that the fullscreen hint is being properly applied.
• Use the EGL window compositor instead, and let menus be natural sizes instead of fullscreen.