bug#13011: 24.2; Text flickering moving cursor with box around text enabled

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 30 Nov 2012 10:13:30 +0200
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 13011@debbugs.gnu.org
> 
> > Date: Fri, 30 Nov 2012 01:45:30 +0100 (CET)
> > From: "mario.giovinazzo@virgilio.it" <mario.giovinazzo@virgilio.it>
> > Cc:  <monnier@iro.umontreal.ca>
> > 
> > The horizontal flickering has 2 cases:
> > 
> > 1) font-lock mode disabled.
> > Current line has a single global box around current line 
> > Moving cursor vertically produce 1 pixel flickering due to the left border 
> > that adds 1 pixel.
> > Moving cursor horizontal (along the same line) produce flickering crossing 
> > parenthesis when paren-mode is enabled. 2 more pixels if the matching 
> > one is in another line, 4 more pixels if on the same. This because it draw 
> > a box on highlight parenthesis adding  2 pixels for box.
> 
> This is the same problem as with stretches of white space.  Its reason
> is separate from the one that causes the entire line to shift one
> pixel to the right when the line thickness is -1.
> 
> > 2) Font-lock-mode enabled.
> > Current line seems to have a single global box but looking careful it has 
> > many consecutive boxes, one for every font-lock-face.
> 
> Same as above: a different reason.

This part of the bug is now fixed in revision 111191 on the trunk.

For the rest of the bug I'm awaiting the decision wrt how to treat
vertical box borders whose width is negative.