bug#23871: 25.1.50; Undo unexpectedly leads to blank buffer

[Eli Zaretskii]

> From: Markus Triska <triska@metalevel.at>
> Cc: 23871@debbugs.gnu.org,  phillip.lord@russet.org.uk
> Date: Tue, 05 Jul 2016 21:47:51 +0200
> 
> In some cases, I would prefer if the window displayed said line at
> its center (instead of at its top).

I thought about this, and doing so is very problematic.  Many of these
cases are when a large change happens in a continuation line which
originally contained the window-start.  When such a change happens,
the display engine cannot know whether the result will be a continued
line or a short line that doesn't need to be continued.  The current
code plays it safe and assumes it will still be continued, and in that
case you don't want the recentering, because it would be unexpected.

Detecting the special case where the remaining line is short enough to
nod need continuation would need significantly more code, with special
cases for invisible text, images, large fonts etc., all of which could
take a line that has just a few character positions and make it need
continuation lines.  So I decided this minor issue isn't worth the
hassle.