zle -M doesn't work properly if the command line fills the screen

zle -M doesn't work properly if the command line fills the screen

[Mikael Magnusson]

If you enter a very long command, and a widget tries to tell you
something, you won't see it, instead the topmost line of the screen is
just scrolled off and then an identical screen is redrawn immediately,
erasing the zle -M message. Completion is also almost useless in this
case, you won't see any of the options, and menu selection can't even
be entered.

reproduce:
% msg() zle -M hello; zle -N msg; bindkey M msg
press alt-10000 a (or make the terminal smaller), then hit M

More generally, it would be nice if you could set the amount of lines
zle will use for a command (even multiline ones); you might not always
want to fill the entire screen.

We already avoid drawing more than fits on the screen by using the
>.... <.... markers, so it should be possible to do the same with less
than the whole screen, but I couldn't quite figure out the code when I
looked at it.

-- 
Mikael Magnusson