bug#56393: Actually fix the long lines display bug

[Stefan Kangas]

Gregory Heytings <gregory@heytings.org> writes:

> As far as I can tell, it's a full and complete solution, which makes a few
> compromises (as few as possible).
[...]
> Font locking is as far as I can see the main reason why Emacs is still a
> bit sluggish in such cases.

Surely, a full and complete solution should make *no* compromises, not
even with secondary features.  (And preferably be completely
transparent to Lisp.)