bug#15957: 24.3.50; Follow mode scrolling broken on Emacs trunk

[Anders Lindgren]

Hi!

I tried something similar to the code you suggested. The code I tried checked both `follow-scroll-up' and `follow-post-command-hook'. It appears as though the selected window is changed somewhere in the post-command hook. This, however, does not occur when I call the post-command hook as a plain function.

Also, I noticed that an old Emacs trunk I had laying around worked correctly, so I have spent some time to do a binary search of the bzr archive and found out that this broke in revision 113753, with the following log message:

revno: 113753

committer: Dmitry Antipov <dmantipov@yandex.ru>

branch nick: trunk

timestamp: Thu 2013-08-08 08:42:40 +0400

message:

  Do not reset window modification event counters excessively.

  These leftovers and poor man's tricky methods to catch extra

  redisplay's attention are no longer needed.

  * frame.c (set_menu_bar_lines_1):

  * minibuf.c (read_minibuf_unwind):

  * window.c (Fset_window_start, set_window_buffer, window_resize_apply)

  (grow_mini_window, shrink_mini_window, window_scroll_pixel_based)

  (window_scroll_line_based, Fset_window_configuration):

  * xdisp.c (redisplay_window): Do not reset last_modified and

  last_overlay_modified counters.

I will continue to narrow down the problem in the post-command hook, I'll let you know when I'm done.

Sincerely,

    Anders Lindgren

On Sun, Nov 24, 2013 at 11:10 AM, martin rudalics <rudalics@gmx.at> wrote:

> I really doubt that the code in `follow-scroll-up' is broken.

I didn't say so.

> Follow-mode
> is designed so that the rearrangement of the other windows (the ones that
> follow the selected window) occur in the post-command hook (to allow
> Follow-mode to act upon all Emacs commands, not only it's special
> function). It appears that something has changed in the display engine, or
> in the way that post-command-hook is called, that makes this mechanism
> fail. This could also account for the difference we see when the function
> is called via a key sequence as compared to via M-x.

IIUC we'd have to find out when and where follow-mode expects the
selected window to be a certain window and why this sometimes fails.  So
maybe you should try the change I suggested.

> This is also the reason why reported this as a bug, rather than digging
> into the code myself. However, I could try to add log code to Follow mode,
> to check if I could try to figure out what is going on.

Please do that.

Thanks, martin