bug#20445: excessive redisplay / echo area resizing during byte-compilation

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Date: Mon, 27 Apr 2015 17:31:28 -0400
> 
> Does not happen with M-x byte-recompile-file.

If you invoke byte-compile-file like this:

  M-: (byte-compile-file "org.el") RET

then it flickers exactly like with the menu-bar invocation.  Which is
expected, since the menu bar invokes emacs-lisp-byte-compile, which
does this:

  (defun emacs-lisp-byte-compile ()
    "Byte compile the file containing the current buffer."
    (interactive)
    (if buffer-file-name
	(byte-compile-file buffer-file-name)
      (error "The buffer must be saved in a file first")))

The difference between "M-x byte-compile-file" and the invocation via
M-: seems to be due to this trick in bytecomp.el:

  (defun byte-compile--message (format &rest args)
    "Like `message', except sometimes don't print to minibuffer.
  If the variable `byte-compile--interactive' is nil, the message
  is not displayed on the minibuffer."
    (apply #'message format args)
    (unless byte-compile--interactive
      (message nil)))

And indeed, if I invoke byte-compile-file like this:

  M-: (let ((byte-compile--interactive t)) (byte-compile-file "org.el")) RET

the flickering goes away.

Now to the source of the flickering: it's the resizing of the echo
area that causes that.  We have this code in echo_area_display:

      window_height_changed_p = display_echo_area (w);
      [...]
	  if (window_height_changed_p <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
	      /* Don't do this if Emacs is shutting down.  Redisplay
	         needs to run hooks.  */
	      && !NILP (Vrun_hooks))
	    {
	      /* Must update other windows.  Likewise as in other
		 cases, don't let this update be interrupted by
		 pending input.  */
	      ptrdiff_t count = SPECPDL_INDEX ();
	      specbind (Qredisplay_dont_pause, Qt);
	      windows_or_buffers_changed = 44;  <<<<<<<<<<<<<<<<<<<<<<
	      redisplay_internal ();

And a non-zero value of windows_or_buffers_changed causes a thorough
redisplay, including, for example, the mode lines, the menu bar and
the scroll bars.

If I set resize-mini-windows to nil, the flickering disappears.

I think what causes the frequent resizes of the echo area are those
calls to (message nil) in byte-compile--message.  Perhaps avoiding
that would resolve the issue.

Otherwise, I guess I'll have to say that this is the display engine
"functioning as designed".

> Does not happen with 24.5, even with the menu-bar.

The trick with byte-compile--message and byte-compile--interactive is
new on master, it didn't exist in Emacs 24.