BTW, you can already try the unified menu if you pass the
--disable-toolbox-menu option to configure. You will have to do this
with a fresh installation though, or things won't work correctly.
Currently this is only done when building on OS X for the Quartz GTK+
backend. But we will want to consider making this the default for all
platforms. However we then absolutely need a window to stick the menu
to. This can probably best be done as outlined in your mockup.
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