bug#36550: mouse-face overlay calculation error

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: 36550@debbugs.gnu.org,  linus.kallberg@outlook.com
> Date: Sat, 13 Jul 2019 15:50:41 +0200
> 
> commit 5d24c60e3a3b07ccb31b886885ea117a058168be
> Author: David Ponce <david@dponce.com>
> Date:   Mon Apr 3 14:34:28 2006 +0000
> 
>     (recentf-open-files-item): Include newline in button
>     field, so opening a file will work, when the point is at the end
>     of the file name.  Allow, for example, to [i]search a file by
>     extension and just push RET to open it.
> 
> So it really wants the widget to have the newline inside the widget for
> usability reasons.

I still don't understand why.  Surely, "the end of the file name" is
before the newline, right?  And what point has to do with that, since
mouse-face is about the mouse pointer, not about point?  What am I
missing here?

> Well, it's not 100% right, since it highlights the first character on
> the next line if you have mouse-face on the preceding newline, which has
> to be a bug even if we're not supposed to put mouse-face on the newline.
> :-)

No, it's 100% right, because it implements a certain set of
requirements, and those mandate that when the end point is beyond the
last character on a line, that end point is in the next line.