bug#14744: 24.3.50; Flickering mouse-face on process output

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> emacs -q
>> C-x 2
>> M-x shell RET
>> cat /dev/urandom RET
>> C-x o
>> # Move mouse on one of the buttons so the face changes to highlight
>> The button flickers here.
>> Xorg and GTK+ 2.24.10 on GNU/Linux and current Emacs trunk.  This issue
>> is easily reproducible on older Emacsen, such as 24.3 or 23.4.

> Redisplay of a window always includes redisplay of the tool bar.  The
> latter involves drawing the buttons, and then applying the depressed
> faced to the button that the mouse pointer hovers above.  That is what
> you see, I believe.  So why do you consider that a bug?

I consider it a bug because some part of the screen flickers.
I understand the underlying technical reason why Emacs does that, but
it's still a misfeature.  Ideally we should refrain from redrawing
things that haven't changed, and the second best fix is to use some kind
of double buffering so the intermediate "drawn but not depressed" state
is kept hidden.


        Stefan