Re: menu-bar: disable items when no frame visible

[Richard M. Stallman]

    Is it possible to automatically signal a `quit' to the current  
    minibuffer prompt function when another minibuffer is set up?

That might be a good feature, especially for beginning users.  It's
not straightforward to implement, since it would require a nonlocal
exit to the command loop, to exit the first minibuffer and the command
that invoked it, and after this it would be necessary to invoke the
second command again.  But it should not be terribly hard.

We should not install this till after the release, but you could
write it now.