bug#22000: Patch addressing the menu-bar frame-resize interaction

[Vivek Dasmohapatra]

> Here's a screenshot showing the the menu and tool bars in Emacs without
> your patches (left), with patches and menu-bar-scrollbar set to
> `automatic' (middle) and set to nil (right):

This first height variation is not a glitch - when a scrollbar is present 
('always) or could be present ('automatic) I have to let GTK reserve
some space for the scrollbar or the focus gets lost after certain
mouse interactions, at least until the user forces it back with a
particular widget interaction (which is very confusing as you
suddenly stop being able to type in any buffers or use the keyboard
to drive that instance of emacs).

When there's no scrollbar (nil) the extra space can be compressed away
with some CSS trickery (in theory the UI focus model is still broken, but 
since there's no scrollbar to interact with the user can't start the bad
interaction sequence).