bug#14838: 24.3.50; repeating next-line or previous-line is broken

[Eli Zaretskii <eliz@gnu.org>]

> From: Stephen Berman <stephen.berman@gmx.net>
> Cc: jan.h.d@swipnet.se,  14838@debbugs.gnu.org
> Date: Thu, 11 Jul 2013 22:04:07 +0200
> 
> > I don't believe the aref part.  I think the real culprit is font-info.
> > Let's conduct an experiment: if you modify default-font-height so that
> > it always just calls frame-char-height, does the problem go away?
> 
> Yep, that it does.  And no trace messages are emitted.

Thanks, I will come up with a change that will refrain from calling
font-info unless it's strictly necessary.

> So it seems some fonts are sensitive to this distinction and others
> aren't.  What is the crucial difference?

Not sure, but I saw that the implementation of font-info actually
opens the font file, so there's probably some non-trivial processing
involved in that.  And likewise for font-face -- it calls the font
back-end.

> There are no trace messages while scrolling is normal; only after
> redisplay stops (with >90% CPU load) and then recovers do the messages
> appear, and they are all as above, with py=0.

Ah, OK, now everything is clear.