Subject: Re: bug#12401: scroll-preserve-screen-position broken with font height faces

From: Le Wang <[email-masked]>
Date: Mon, 10 Sep 2012 20:37:06 +0800

Thanks for the quick response, Eli.

On Mon, Sep 10, 2012 at 3:23 PM, Eli Zaretskii <[email-masked]> wrote:
> scroll-preserve-screen-position tries to preserve the
> _screen_position_ of point, i.e. where you find the cursor, after it
> executes one of the scrolling commands.  But since arbitrary pixel
> coordinates on the screen might in general (in the presence of
> variable-size fonts) correspond to one of 2 possible text lines, Emacs
> needs to choose where to put the cursor.  What exactly do you expect
> Emacs to do in such a situation, where it simply _cannot_ keep the
> same pixel coordinates?

I understand the problem.

> IOW, what is the "contract" that you expect Emacs to keep under
> scroll-preserve-screen-position?  Is it just that doing N C-v's
> followed by N M-v's brings point to the same location where it was
> before this sequence of 2N commands?  Or do you also expect something
> from each individual C-v/M-v, and if so, what exactly?

The contract should be C-v followed by M-v should bring the point back
to the same line.  If the max_column after C-v is less than
current_column, then either restoring the column as current_column or
max_column is acceptable.

Although the current behaviour of C-p followed by C-n seems to restore
current_column, so that would be more consistent.

As a corrolary, if this contract is enforced then N C-v followed by N
M-v should also bring point back to the same line.

>> For reference this package works just fine with variable height faces:
>> http://www.cs.utah.edu/~eeide/emacs/scroll-in-place.el.gz
>
> I cannot use it, neither in Emacs 24.2 nor in the current trunk code:
> it complains about a missing function screen-width.  Please provide a
> complete recipe for using this package, starting with "emacs -Q", in
> Emacs 24.2 or newer, so I could study what it does and how.

Sorry about this.  It seems the version I use comes from XEmacs and
I've made a small fix.  I've placed it here:

https://gist.github.com/3690403

> For that matter, please also provide all the details collected by
> report-emacs-bug, they might be important.

The repro steps I've described starts from "emacs -Q".  I can still
provide this information, if needed though.

-- 
Le