bug#736: C-l interacts badly with scroll-conservatively

["Drew Adams" <drew.adams@oracle.com>]

> >> emacs -q
> >> M-: (setq scroll-conservatively 100) RET
> >> C-h n
> >> C-v
> >> C-l
> >> C-l
> >> C-l
> >> 
> >> The repeated C-l's don't move to the top or bottom of the 
> >> window, unlike the usual behavior (new to Emacs 23) of
> >> recenter-top-bottom.
> >
> > Maybe I'm misunderstanding the bug report, but by design
> > `scroll-conservatively' modifies the behavior of
> > `recenter-top-bottom'. As the doc string says:
> >
> >   "Top and bottom destinations are actually
> >    `scroll-conservatively' lines from true window top
> >    and bottom."
> 
> That's true; I didn't see that.
> 
> But why does recenter-top-bottom use scroll-conservatively?  AFAICT,
> people often use a large nonzero number for scroll-conservatively so
> that when point is at the bottom of the window, C-n scrolls 
> down by one
> and keeps the cursor at the bottom; and similarly, if point is at the
> top of the window, C-p keeps the cursor at the top.  It doesn't make
> sense to make scroll-conservatively affect C-l.
> 
> I think we should use scroll-margin for this purpose, rather than
> scroll-conservatively.
> 
> What do people think?

I believe this (below) was the last message in the thread that discussed this.
AFAIK, nothing was decided about using a different user option or rationalizing
the various uses and doc for `scroll-conservatively'.

---------------8<---------------------------
> From: Drew Adams Sent: Saturday, December 01, 2007 8:54 PM
> To: emacs-devel@gnu.org Subject: RE: recenter-top-bottom
> 
> > the doc string of scroll-step says this:
> > 
> >     "If you want scrolling to always be a line at a time, 
> >      you should set `scroll-conservatively' to a large
> >      value rather than set this to 1."
> 
> Where "this" refers to `scroll-step', not 
> `scroll-conservatively', presumably. That could be made 
> clearer, BTW. And "a large value" could be made clearer also: 
> example? What's large?
> 
> > When scroll-conservatively is changed, it's usually for 
> > this reason, I think. (See Richard's explanation.)
> 
> 1. Then change the `recenter-top-bottom' code to use a new 
> user option, or some other existing option that is more 
> appropriate than `scroll-conservatively'.
> 
> 2. FWIW, the manual, the `scroll-step' doc string, and 
> Richard seem to be saying different things. The manual 
> (Emacs, node Auto Scrolling) says that you can use "a small 
> number" for `scroll-conservatively' to scroll "the text just 
> far enough to bring point back on screen". The doc string of 
> `scroll-step' says that you can use "a large value" for 
> `scroll-conservatively' to scroll "a line at a time". Those 
> can both be true, but they would seem to be different uses of 
> the option. 
> 
> Richard says that he sets `scroll-conservatively' to a large 
> value to "bring point onto the screen by scrolling, with even 
> one line of overlap". That doesn't seem to be the same thing 
> as scrolling "a line at a time".
> 
> I'm not saying that anyone is wrong or that the doc should be 
> changed (I don't know). And I'm no longer claiming that 
> `scroll-conservatively' is appropriate for 
> `recenter-top-bottom' (use some other option, if you like). 
> I'm saying only that the uses or interpretations of 
> `scroll-conservatively' seem various.