bug#12867: 24.3.50; easy-to-repro crash involving mode line

[Stefan Monnier]

> We already do, but maybe not as extensively as you may have in mind.
> See the base_line_pos member of 'struct window'.
> There's also region-cache.c, which we could use for that.  But before
> we do, Someone(TM) should time the thing and see if it's worth our
> while to make that cache.

FWIW, I recently added such a cache (in Elisp) to nlinum.el, and on my
test case (largish file with C-v/M-v style scrolling) the difference was
very significant (but the previous code had no cache at all, not even
base_line_pos).


        Stefan