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 -QEnter 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 3M-: (set-window-start (selected-window) (point-max)) RETC-x oC-pHere, 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 LindgrenOn 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 -QC-h tM->M-x follow-delete-other-windows-and-split RETC-pC-pAfter 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 LindgrenOn 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 -QC-x 3C-x oM-: (set-window-start (selected-window) (point-max)) RETC-x oM-<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.-- AndersOn 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> always tries ensure that windows are not empty so it repositions it... So,
> 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
> 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