Re: Scrolling jumpy when line-spacing > 0

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuri Khan <yuri.v.khan@gmail.com>
> Date: Fri, 21 Apr 2017 17:13:32 +0700
> 
> (defun yk-scroll-up ()
>   (interactive)
>   (scroll-up 1))
> (defun yk-scroll-down ()
>   (interactive)
>   (scroll-down 1))
> (global-set-key (kbd "C-<up>") 'yk-scroll-down)
> (global-set-key (kbd "C-<down>") 'yk-scroll-up)
> 
> This works so far.
> 
> Next, I also want my lines spaced a bit more widely.
> 
> (setq line-spacing 0.5)
> 
> And this is where I have a problem.

(Why do people report bugs here, rather than with report-emacs-bug?)

> $ emacs -Q
> M-x emacs-version RET
> GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.18.9) of
> 2016-04-17 on lgw01-04, modified by Debian
> M-x view-emacs-news RET
> M-x set-variable RET line-spacing RET 0.5 RET
> M-x split-window-vertically
> 
> Use the mouse to drag the top window’s modeline so that it is
> immediately (two pixels or so) after the line, “See files NEWS.23,
> NEWS.22, NEWS.21, NEWS.20, NEWS.19, NEWS.18,”.
> 
> Position the point on that line, using a mouse click.
> 
> M-: (scroll-up 1)
> 
> Observed behavior: Point is on the same line and the line is at the
> top of window.

I can see this in Emacs 24.5 and in Emacs 25, but not on the master
branch.  On the master branch, when you click on the bottom line of
the window, which is only partially visible in the above scenario,
Emacs scrolls the window half-window up.  To be able to put point on
the bottom line, you will need to set window-resize-pixelwise non-nil,
and drag the mode line down pixel by pixel until Emacs lets you put
point there.  Once this happens, "M-: (scroll-up 1) RET" produces the
effect you expected.

What happens in Emacs 25 and older is that Emacs decides that the
window-start is no good and needs to be recomputed.  It then makes
point the window-start point, and that has the effect of a full window
scroll.