bug#29325: 26.0.90: Info scrolling stuck

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 20 Nov 2017 20:52:39 +0100
> From: charles@aurox.ch (Charles A. Roelli)
> CC: 29325@debbugs.gnu.org
> 
> I don't have access to a GNU/Linux system at the moment, but if I
> recall correctly, it would be 5 lines at a time, when scrolling
> slowly.

Then I'd expect the same behavior to happen if you do "M-5 M-v"
instead of turning the wheel.  Does it?

If M-v doesn't reproduce the problem, then what about removing the
tricky code, and leaving just

  (funcall mwheel-scroll-down-function amt)

without all the rest?  I cannot understand why we need the rest, maybe
I'm missing something.  mwheel-scroll-down-function is by default
scroll-down, exactly the same function called by M-v, so the results
should be the same, assuming that 'amt' is 5 when turning the wheel.

> The behavior is different depending on how fast the mouse wheel is
> turned.  If it is turned slowly several times in a row, it is possible
> to make window-start reach as far as the second or third line of the
> buffer.  If you turn it once more, then window-start jerks down a few
> lines instead of reaching the start of the buffer.  This can be
> repeated ad infinitum.
> 
> In contrast, when turning the mouse wheel quickly (a few notches in
> one second), it is not possible to get window-start anywhere near the
> start of the buffer, except by using non-mouse commands or by setting
> point closer to the top of the window first (with the mouse).

Just to make sure we are not barking up the wrong tree: this is in
"emacs -Q" with no customizations of any scroll-related variables,
like scroll-margin, scroll-conservatively, etc., right?