bug#19650:

[Stefan Monnier]

> 1) Changing insert for insert-before-markers is problematic because it
> somehow wreaks havoc with font locking. I guess some other markers are
> being moved around besides he-string-end, maybe the one pointing to
> the end of the prompt or something like that (since everything gets
> colored the same the prompt is). Moreover, the he-string-beg marker
> would still point to the wrong place no matter what its type were.

Indeed, the issue is not "insert-before" vs "insert-after" but that we
delete+insert the exact same text, so the markers should simply not
move, but Emacs can't know that.

Basically, the delete+insert only changes the text's properties, so "the
right way" to fix this is to not delete and not insert, but instead to
take the result string, extract its text-properties and apply them
carefully to the corresponding of the buffer, so as not to affect
any marker.

Another "right way" is to do the font-locking "in situ" instead of doing
it in a side buffer.  This would be good, but IIUC it's not as
straightforward as it sounds.

> 3) Same as (2) above, but saving/restoring the *positions*  of both
> he-string-beg and he-string-end. This is the best solution I could
> figure out till now. Take a look at the attached patch, please.
> What do you think?

I think it's hideous, but maybe it's OK as a temporary workaround.
I'll let Fabian decide if he wants it in his code.  But if he accepts
it, the code should have some extra comments explaining what's going on.


        Stefan