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

[Markus Triska <triska@metalevel.at>]

phillip.lord@russet.org.uk (Phillip Lord) writes:

> I haven't started to look at 23871 yet, only 21722; I would rather deal
> with the simple case first. Can you tell me whether it fixes this
> case?

The patch fixes the concrete case reported in #21722 but leads to a new
problem with undo that is not there without this patch. I cannot regard
this as a good fix for #21722. If you want to close #21722 after
applying your patch, I can file a new report for the new problem.

Please note that #23871 is the main problem for me and has been present
in Emacs for about a decade. The other issues I filed are only my
attempts to make the issue more easily reproducible, and may in fact
even be unrelated issues that I find while trying to find the root cause
of #23871. I first filed #1095, then adapted this to #21722, and now
also filed #23871 because redisplay may also be involved.

I have tolerated the problems described in #1095 and #21722 for about a
decade and see no reason to delay Emacs 25: Neither Emacs 23 nor 24 were
delayed because of them although these issues were already reported!
#23871 was also present during this time but not yet reported.

All the best,
Markus