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

From: Anders Lindgren
Subject: bug#16129: 24.3.50; Emacs slow with follow-mode when buffer ends before last window
Date: Thu, 2 Jan 2014 14:55:59 +0100

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-x follow-delete-other-windows-and-split RET

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.

    Anders Lindgren

On Fri, Dec 13, 2013 at 6:55 PM, Anders Lindgren <address@hidden> wrote:

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
     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 <address@hidden> 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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