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

[Anders Lindgren]

Hi!

No, the actual numbers don't appear to be important. I simply picked some numbers to make sure the frame was wide enough for two side-by-side windows. I tried 40 instead of 70 and you get the same effect.

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

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.

    -- Anders

On Sat, Nov 23, 2013 at 12:58 PM, martin rudalics <rudalics@gmx.at> wrote:

Hi Anders

> Scrolling multiple pages when Follow mode is enabled sometimes fails.
>
> Steps to repeat:
>
>     Emacs -Q
>     C-h t
>     ESC : (set-frame-size (selected-frame) 200 70) RET

Is this really necessary?  70 lines means I can't see my echo area any
more.

>     M-x follow-delete-other-windows-and-split RET
>     C-c . C-v
>
> The intent is to scroll the entire document two pages down, the left
> window should still be selected (this causes Follow mode to arrange the
> right window after it). Instead, the right window is selected,

Can you tell when and where the window on the right is selected?

> which causes
> the left to be arranged after it, effectively undoing the scroll.

IIUC `follow-scroll-up' should select the proper window via

         (let* ((windows (follow-all-followers))
                (end (window-end (car (reverse windows)))))
           (if (eq end (point-max))
               (signal 'end-of-buffer nil)
             (select-window (car windows))

which seems to imply that `follow-all-followers' doesn't return what it
is supposed to.  Am I correct that the first window returned by the
latter should be `frame-first-window'?  In that case try to replace the
last line of `follow-all-followers' by something like

    (prog1
        (setq windows (sort windows 'follow--window-sorter))
      (unless (eq (car windows) (frame-first-window (window-frame win)))
        (error "Bad windows %s" windows)))))

and maybe this way we can find out whether the problem is there.

> The problem seems to be sometimes intermittent, sometimes the scroll works,
> sometimes it don't.
>
> An interesting effect is that the problem only occurs when
> follow-scroll-up is bound to a key. When issuing M-x follow-scroll-up
> RET, the command seems to be working properly.

I have no idea why this could change the course of things because ARG
should be nil in both cases.

> By the way, I'm the original author of Follow mode, even tough I haven't
> touched it for many years.

I suppose you should touch it again ;-)

martin