bug#43587: move-to-column behaves differently when text has invisible property

[Eli Zaretskii]

> Date: Tue, 29 Sep 2020 22:00:24 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 43587@debbugs.gnu.org
> 
> It's a deficiency in the algorithm used by move-to-column that
> determines whether the goal column is in the middle of a TAB.  It
> assumes that in such a case the TAB is the previous character that
> ends at the column where the movement ended, but that is not so when
> invisible text immediately follows the TAB, because moving over the
> TAB will in that case also skip all of the following invisible text.
> 
> I will see how this can be fixed without making the function too slow
> (because it needs to consider overlays as well, not just text
> properties).

I think I fixed this now on the master branch.