Re: animate incredibly slow compared to 21.3

[Richard Stallman]

    > The display mechanism at C level is already designed to facilitate
    > this.  It's just a matter of adapting the higher levels to use it.

    I don't understand -- what C level "display mechanism" is designed to
    facilitate "this".

There are many functions in xdisp.c that operate on iterators.
Many are called move...

    The two others can be replaced by MUCH SIMPLER code using
    posn-at-point, like this:

If these jobs can be done with posn-at-point, that's fine.
posn-at-point is a subset of compute-motion; it works only
within the screen height, and only goes from a buffer position
to a screen position.

    If there are trivial replacements for the existing uses, it's not
    "essential".

    Actually, these new versions works better with proportional fonts than
    the old code using compute-motion.

Of course it does--because compute-motion has not been updated to
handle proportional fonts.  That's the change I have been asking for
for 4 years now.

posn-at-point is essentially a special case of compute-motion.
posn-at-point does its job by calling pos_visible_p, which uses
start_display, move_it_by_lines, and move_it_to.  That's how
compute-motion could work, too.