bug#36550: mouse-face overlay calculation error

[Eli Zaretskii <eliz@gnu.org>]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Sat, 13 Jul 2019 02:31:15 +0200
> Cc: "36550@debbugs.gnu.org" <36550@debbugs.gnu.org>
> 
> (progn
>   (let ((point (point)))
>     (insert "foo\n")
>     (let ((o (make-overlay point (point))))
>       (overlay-put o 'mouse-face 'highlight)
>       (insert "bar"))))
> 
> This should make a mouse face that's displayed the entire "foo" line,
> but it extends to the first character of the next line.
> 
> If you make it one character shorter, then the entire line isn't
> highlighted.
> 
> And!  If you say `face' instead of `mouse-face', then everything is
> highlighted correctly (i.e., just the entire "foo" line, and not the "b"
> on the next line).

Mouse-face isn't supposed to cover newlines, I think.  Why do you need
that?

The "one character shorter" variant does what it's expected to do,
because mouse-face is not extended to EOL as with other faces.
Mouse-face is for showing the parts of text that are mouse-sensitive,
so it makes no sense to highlight portions of display that have no
text.

> So is there some basic fault in the code that calculates the length of
> the mouse highlighting?  I don't really know where to start looking...

It's in the display code, and is quite complicated due to
bidirectional text use case.  See mouse_face_from_buffer_pos and its
subroutine rows_from_pos_range.