Subject: bug#21092: 25.0.50; Option `lazy-highlight-max-at-a-time' does not work

From: Drew Adams <[email-masked]>
Date: Fri, 28 Aug 2015 08:59:34 -0700 (PDT)

> > IIUC, a lot has changed since lazy highlighting and these options
> > were first introduced - jit-lock, etc.  Perhaps this worked at one
> > time, and a regression was introduced since then?  (No, I don't see
> > that myself - the same bug is in Emacs 22.3.)
> 
> Do you see any previous version of Emacs where it worked as you
> expect?  I don't.

As I said, no, I don't either.

I am also guessing that few people have ever used a nil value of
`lazy-highlight-cleanup'.  I have it on/off as an Isearch toggle
key now, and with that easy access I find it quite useful.

> I will now shut up and let people who actually know something about
> isearch.el talk.

OK, but I am interested in the general question of whether the
display engine precludes highlighting more than what is "on screen".

Could you speak to that?  What am I missing?  AFAICT, it is possible
to highlight, with either text properties or overlays, a very large
buffer.  Do you agree?

If so, why should that not be possible for lazy highlighting also,
because of a display-engine limitation/design?  IOW, I don't
understand how I can highlight a large buffer but Isearch
lazy-highlight cannot do this because of the display engine.

This is not a rhetorical argument.  I would like to understand
whether it is possible to fix this bug, to let users control
the scope of lazy highlighting beyond what is "on the screen".

Perhaps neither you nor I "know something about isearch.el", but
you certainly know something about the display engine.  A little
help understanding this limitation, please.  Thx.