bug#26445: 26.0.50; Scroll margin and cursor movement working incorrectly when scrolling over different height lines

[Alexander Miller <alexanderm@web.de>]

This is probably a continuation of bug#25792.

The issue is that scrolling downwards with a scroll-margin
has the cursor jumping again - this time the issue is caused by 
different height
lines. This does not simply mean being inside a buffer where some lines
are fontified to use a different height attribute than others. The
stutter already occurs due to the miniscule height differences
introduced by box or over- and underline face attributes.

The following bits of code can reproduce the issue in emacs -q:

* (custom-set-faces `(button ((t (:box "#ffffff" :underline nil 
:overline nil
:background nil :foreground "#ffffff")))))
* (setq scroll-margin 10 scroll-conservatively 101)
* (defun maketext ()
(interactive)
(dotimes (i 1000)
(insert (format "\ntext %s\n" i))
(insert-text-button (format "text %s" i))))
* (maketext)

Scrolling down in this text induces the same stutter as in the previous
bug. Strangely enough the issue only occurs when the cursor is in a
column other than the very first. Only scrolling downwards is affected,
at least in a way I was able to reproduce it in emacs -q.
I also do have the same issue with upwards scrolling in org-agenda buffers
when running a heavily modified spacemacs, with a theme that makes heavy
use of boxed and different height text, so it's difficult to say where
things start exactly.

The second, maybe wholly unrelated, part is the cursor's column
movement. This time scrolling in either direction is affected. To
reproduce put the cursor at the very end of a line containing
unboxed text and scroll down or up. The cursor will move to the previous
column on every boxed line and move back again for unboxed lines.


In GNU Emacs 26.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.22.10)
of 2017-04-10 built on a-laptop
Repository revision: 3ccd0ff1064a2836c379b13c2d5f4b11c5da1f88
Windowing system distributor 'The X.Org Foundation