GNU bug report logs - #15957
24.3.50; Follow mode scrolling broken on Emacs trunk

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From: martin rudalics <rudalics <at> gmx.at>
To: Anders Lindgren <andlind <at> gmail.com>
Cc: 15957 <at> debbugs.gnu.org
Subject: bug#15957: 24.3.50; Follow mode scrolling broken on Emacs trunk
Date: Sat, 23 Nov 2013 12:58:47 +0100

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

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