bug#21869: Redisplay: after echo area diminishes in size, Follow Mode windows aren't resynchronised.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

With a recent copy of the git master, in either X windows or a Linux tty,

emacs -Q

If necessary, make the window wide with the mouse.

C-x C-f src/xdisp.c       ; or any other non-small file.
C-x 3
M-x follow-mode

Note the content of the lines around the window division.

M-: (message "x\ny\nz") <CR>

The echo area expands, and the two Follow Mode windows get correctly
synchronised.  Again, note the content of the lines around the window
division.

C-f

The echo area shrinks back to its normal size.  Note the content of the
lines around the window division.  The two lines at the bottom of the
left hand window are also displayed at the top of the right hand window.
The windows have NOT been synchronised after the change in the echo area
size.  This is a bug.

#########################################################################

Tentative analysis:

Follow Mode puts a function on window-scroll-functions which should be
called any time a window changes size.  At the C-f above,
window-size-change-functions is not being invoked.

The one single place where window-size-change-functions is invoked is in
the function prepare_menu_bars in xdisp.c.

The one single place where prepare_menu_bars is called is fairly early on
in redisplay_internal, at L+137.  This is before the echo area is resized
at L+164.  Thus, the check on changed window sizes happens too early,
before the change in the echo area size.

-- 
Alan Mackenzie (Nuremberg, Germany).