Removing redisplay-dont-pause

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> branch: scratch/tty-child-frames
>> commit f62d70f52f4f6b7ed158d618bf790df21f171172
>> Author: Gerd Möllmann <gerd@gnu.org>
>> Commit: Gerd Möllmann <gerd@gnu.org>
>>
>>     Don't pause display for pending input
>>
>>     * src/dispnew.c: Remove display_completed, redisplay_dont_pause,
>>     redisplay-dont-pause was declared obsolete in Emacs 24.  Remove anything
>>     checking pending input, change function signatures accordingly, and so
>>     on.
>>
>>     * src/keyboard.c (read_char): Don't use redisplay_dont_pause.
>>     * src/minibuf.c (read_minibuf): Use new function signatures.
>>     * src/xdisp.c: Don't check display_completed. Use new API.
>>
>>     * lisp/subr.el (redisplay-dont-pause): Remove declaration.
>
> I don't think this kind of change is appropriate.  Feature branches
> should not add/remove features not directly related to the feature
> being developed on the branch.  If we want to remove
> redisplay-dont-pause from Emacs (and I'm not yet sure we do), it
> should be discussed on emacs-devel or in a dedicated bug report, not
> silently installed on the branch.

I guess not many know what this is about, so what is this about?

One feature of the old redisplay was that it stopped updating the
display when it detected that input was pending, so that it could
process that input ASAP.

I kept that feature in my redisplay. As a debugging aid, I added
redisplay-dont-pause with which I could turn this feature on and off,
because pausing the display lead to a lot subtle and difficult to debug
bugs.

ISTR that setting redisplay-dont-pause to t became sort of a secret
semi-popular hack. Hyrum's Law in action, I guess. And 13 years ago or
so, the default of redisplay-dont-pause was changed to t by Eli.

10 years ago, Stefan Monnier explained the why

  +  /* Contrary to expectations, a value of "false" can be detrimental to
  +     responsiveness since aborting a redisplay throws away some of the
  +     work already performed.  It's usually more efficient (and gives
  +     more prompt feedback to the user) to let the redisplay terminate,
  +     and just completely skip the next command's redisplay (which is
  +     done regardless of this setting if there's pending input at the
  +     beginning of the next redisplay).  */
  +  redisplay_dont_pause = true;

And finally, Stefan also

  (make-obsolete-variable 'redisplay-dont-pause nil "24.5")

and removed it from the docs.

I've removed that pausing feature in my fork and thought I'd bring that
to master via scratch/tty-child-frames, but apparently that's
controversial for some reason I don't understand, and I reverted that.
And so, here we are.