bug#25978: 25.1; Message from `text-scale-adjust' is shown in other frames too

[Drew Adams <drew.adams@oracle.com>]

> > > > Here I see it only in the frame where I invoked "C-x -".
> > >
> > > After you see it there, click the title bar of the other frame,
> > > to select it.  The message is moved to that (selected) frame's
> > > echo area.  But using +, 0, or - in that frame does not have
> > > the effect of the message.  That frame shows a different buffer,
> > > so +, 0, or - there is governed by the keymap of that buffer's
> > > frame.
> >
> > Not sure what should happen instead (i.e., the proper fix).
> 
> As long as "C-x C--" uses 'message' to display these instructions,
> nothing can be done, because what you see is the usual behavior of
> messages displayed in the echo area: they follow the selected frame.
> 
> Perhaps the frame-switch event should simply empty the echo area.
> Patches to that effect are welcome.

Frame-switch should not systematically empty the echo area, IMO.

Perhaps there is no easy way to fix the problem for this scenario
or similar (anytime the message makes no sense with a different
frame selected).  Dunno.

But note that the problem does not occur when a different window
(not frame) is selected.  For example, with a single frame with
two windows showing two buffers, `C-x C--' in one window shows
the message, but as soon as you select the other window (e.g.
clicking in it) the message goes away and the scaling command
is quit (e.g. selecting the window with the scaled text again
does not redisplay the message).

Feel free to close this bug, if you don't see a solution.
(I don't have a patch to fix it.)  Or perhaps leave it open,
in case it can be fixed in the future.  Ideally, selecting a
different frame should be handled, I think, similarly to 
selecting a different window.