Re: "... the window start at a meaningless point within a line."

[Alan Mackenzie <acm@muc.de>]

Hello, Eli

On Thu, Oct 01, 2015 at 03:03:09PM +0300, Eli Zaretskii wrote:
> > Date: Thu, 1 Oct 2015 11:02:04 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>

[ .... ]

> > This horizontal scrolling (of a long line at the top of W3, when a
> > scroll-down brings the beginning of the line onto the window) would be
> > precisely what's wanted here.

> Then go ahead and make the change.  The display engine will cope.  I'm
> not sure what users will say, but that's not my problem here ;-)

Underway.  Thus far, I've managed to get all the characters in the broken
line displayed "correctly".  That was by adding the extra optional
parameter to set-window-start, an extra field to struct window, and
testing for it in compute_window_start_on_continuation_line.  I can
scroll the windows, mostly, and this works.  So far, so good.

But vertical_motion doesn't work properly.  (vertical-motion 0) puts
point where Emacs thinks the line start ought to be, not where it
actually is.  I'm looking into this.  There may be quite a few places in
the code like this.

[ .... ]

> > Another (lesser) problem is moving point from W2 to W3 with C-x o
> > sometimes causes W1 and W2 to scroll up one line; this has the same
> > cause.  It is suboptimal.

> IIUC, you will not be able to fix this.  Changes near the boundary
> between 2 windows will always be likely to cause some scrolling like
> that.

:-)

> > > > If we were to go this route (of repositioning to avoid line breaks
> > > > between follow windows), there would have to be a limit on how far one
> > > > could scroll, with a value such as 3.

> > > In what units?  Screen lines?  Why only 3?

> > Yes, 3 screen lines.  With a command like `scrolldown-n', or C-u 1
> > <PageUp>, the user is requesting a single line scroll.  Scrolling more
> > than this, even 2 or 3 lines, would be puzzling and irritating.

> Not sure if horizontal scrolling will be more or less irritating, but
> I guess we will find out soon.

We will, hopefully.

-- 
Alan Mackenzie (Nuremberg, Germany).