ediff maximum line length?

[Giorgos Keramidas]

ediff-mode highlights nicely all fine diffs within lines, using a
different face and this has proven *very* useful in many cases, but I've
now bumped on a case (and it will be often the case with some of the
stuff I'm lately doing) where ediff does not display fine diffs for
*very* long lines...

I did try setting up `ediff-auto-refine-limit' to a larger value, e.g.:

    (setq-default ediff-auto-refine-limit 16000)

But then the entire line is marked as 'fine diff' when I try for example
to diff the following files:

    perl -e 'print "x" x 15000 . "\n"' > file-a.txt
    perl -e 'print "x" x  8000 . "yyy" . "x" x 6977 . "\n"' > file-b.txt

Is the behavior of highlighting the full line as fine diff bogus when I
have configured `ediff-auto-refine-limit' to 16000?

If this is not a bug, is there any way to really highlight only the
"yyy" characters in the middle of such a huge line?