bug#16129: 24.3.50; Emacs slow with follow-mode when buffer ends before last window

[Anders Lindgren <andlind@gmail.com>]

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Hi again!

I've dug a bit more into this. It looks like it's not follow-mode that
repositions the window, instead I guess it's some kind of recentering code
in the display engine that have gone "crazy".

I've managed to reproduce the problem with an extremely cut-down version of
the code, which simply reads and restores window-start of windows when
windows-start is point-max.

To reproduce, do the following:

    emacs -Q
    Enter and evaluate the following:

(defun my-avoid-tail-recenter (&rest _rest)
  (let* ((orig-buffer (current-buffer))
 (top (frame-first-window))
 (win top))
    ;; If the only window in the frame is a minibuffer
    ;; window, `next-window' will never find it again...
    (unless (window-minibuffer-p top)
      (while
  (let ((start (window-start win)))
    (set-buffer (window-buffer win))
    (if (eq (point-max) start)
;; Write the same window start back, but don't
;; set the NOFORCE flag.
(set-window-start win start))
    (setq win (next-window win 'not t))
    (not (eq win top))))  ;; Loop while this is true.
      (set-buffer orig-buffer))))

(add-hook 'post-command-hook 'my-avoid-tail-recenter)

    C-x 3
    M-: (set-window-start (selected-window) (point-max)) RET
    C-x o
    C-p

Here, the window will be recentered without there being any reason for it.

Note that this will apply to any window in any frame, as long as there is a
window where window-start == point-max, even if the window displays a
different buffer.

As this now falls outside of follow-mode, I think that I have reached the
end of what I can contribute...

Sincerely,
     Anders Lindgren



On Thu, Jan 2, 2014 at 2:55 PM, Anders Lindgren <andlind@gmail.com> wrote:

> Hi again!
>
> In addition to the problems I originally reported, I realized today that
> the modification also made follow-mode place windows incorrectly, which
> indicates that some primitive display-related function returns incorrect
> values.
>
> Do you want me to report a new bug, or should we see this as a second
> symptom?
>
> You can verify this by doing the following steps:
>
>     emacs -Q
>     C-h t
>     M->
>     M-x follow-delete-other-windows-and-split RET
>     C-p
>     C-p
>
> After the second C-p, the left window is recentered, which is shouldn't.
> This typically occurs when follow-mode thinks the point is not visible in
> any window, which probably is due to incorrect values being reported from
> primitive functions. (For example, in bug #15957 `window-end' didn't honour
> it's FORCE argument, since some display functions didn't mark the end value
> as being dirty.)
>
> I will try to track this down in more detail. However, I wanted to give
> you a heads up since it's appears as though you are close to a release --
> it might take me a couple of days to find the problem, as I have very
> limited time to spend on Emacs-related things.
>
> Sincerely,
>     Anders Lindgren
>
>
>
> On Fri, Dec 13, 2013 at 6:55 PM, Anders Lindgren <andlind@gmail.com>wrote:
>
>> Hi!
>>
>> I agree that we would need to find out why the patch makes Emacs slow.
>> (In fact, I only supplied the information about the internals of
>> follow-mode to help you track down the problems with the slowdown.)
>>
>> However, I don't agree with Eli -- it is possible to place window-start
>> at point-max! However, there is code in the display engine that explicitly
>> recenters such windows, after a while, or when something happens. For
>> example:
>>
>>      emacs -Q
>>      C-x 3
>>      C-x o
>>      M-: (set-window-start (selected-window) (point-max)) RET
>>      C-x o
>>      M-<
>>      blablabla     (type some text)
>>
>> As you type text in the left window at the beginning of the scratch
>> buffer, the right window is recentered. Follow-mode needs its windows to
>> stay put (even the empty ones), as this is essential in creating the
>> illusion that a number of windows make up a very tall virtual window.
>>
>> When I originally wrote follow-mode (some 18 years ago), I suggested to
>> the Emacs maintainers to add a feature to make the recentering of empty
>> windows conditional, so that follow-mode could control this. However, at
>> the time they were not interested so I continued with the current system,
>> which has worked flawlessly since then.
>>
>> If you are interested in making the change in the display engine,
>> follow-mode should of course be rewritten to use it. Otherwise, I suggest
>> that we keep it as it is today -- solutions using overlays etc. don't
>> appeal to me at all.
>>
>>     -- Anders
>>
>>
>>
>> On Fri, Dec 13, 2013 at 5:38 PM, Stefan Monnier <monnier@iro.umontreal.ca
>> > wrote:
>>
>>> > I am the original author of follow-mode, so I can share one interesting
>>> > implementation detail. When the viewed buffer ends before the last
>>> window,
>>> > follow-mode tries to display this window without any content (by
>>> setting
>>> > the window start to point-max). Unfortunately, the Emacs display engine
>>> > always tries ensure that windows are not empty so it repositions it...
>>> So,
>>> > follow-mode hammers in its view of the world every chance it gets,
>>> > currrently in post-command hook and window-scroll-functions.
>>>
>>> Hmm.. so we have 2 things to do:
>>> 1- figure out why my patch slowed things down so much.
>>> 2- change follow-mode to use a different approach.  Maybe a good way is
>>>    to do the following: put window-point at point-max, and add an overlay
>>>    on window-start...point-max that makes the text invisible (with
>>>    a `window' property, so it's only invisible in that window).
>>>    Of course, maybe that won't work either.  But hooking everywhere
>>>    doesn't sound like a good idea.
>>>
>>>
>>> -- Stefab
>>>
>>
>>
>

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