Re: line-start?

[Eli Zaretskii <eliz@gnu.org>]

> From: Bastien <bzg@gnu.org>
> Cc: emacs-devel@gnu.org
> Date: Fri, 07 Feb 2020 11:11:15 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Once again, window-hscroll just returns the number of columns by which
> > the window is hscrolled, it doesn't return the buffer position.  For
> > example, window-hscroll will return the same value for each line in
> > the window, although the buffer position of the beginning of each line
> > is different.  Are you looking for columns or for buffer positions?
> 
> I was looking for columns.

Then (nth 1) of what posn-at-x-y returns is not what you want.  You
should use the (car (nth 6)) instead.

> The use case was this one: given a line, display the content of this
> line in the header line.  When horizontally scrolled, the header has
> to display a substring of the line under the cursor, something like
> (substring (current-line-string) offset-in-columns).

But using the buffer position, you could use buffer-substring instead,
right?

> (defun window-line-start (&optional pos window)
>   "Return the position of the visual start of the line.
> POS defaults to point in WINDOW; WINDOW defaults to the current window."
>   (nth 1 (posn-at-x-y
> 	  0 (+ (cdr (posn-x-y (posn-at-point pos window)))
> 	       (- (nth 1 (window-edges nil t nil t))
> 		  (nth 1 (window-edges nil nil nil t)))))))
> 
> My use-case is gone, I went with overlays instead of the header line.
> 
> But I guess `window-line-start' could perhaps be useful to others.

Maybe.