bug#57150: 29.0.50; [PATCH] Add test coverage for overlay modification hooks

[Eli Zaretskii <eliz@gnu.org>]

> From: Richard Stallman <rms@gnu.org>
> Cc: yantar92@gmail.com, matt@rfc20.org, 57150@debbugs.gnu.org
> Date: Tue, 16 Aug 2022 22:50:38 -0400
> 
>   > The problems with overlays in Emacs are much wider than just Org.  Our
>   > performance sucks in many cases because of that.  Just two examples
>   > that were mentioned lately in the "long-line optimizations"
>   > discussion: show-paren-mode and isearch-lazy-highlight -- each one of
>   > these two is capable of literally bringing Emacs to its knees,
>   > response-time wise.
> 
> The first question is, should these use text properties instead of
> overlays?

Text properties are less convenient when used for ephemeral
highlighting.

> Are they using many overlays to highlight many potential matches in a
> long buffer?

isearch-lazy-highlight does.

> If so, would it make sense to show, at any time, only those
> that are near point?  When you move to a far-away part of the buffer,
> it could add highlighting to the matches near there.  But maybe it
> can avoid ever trying to highlight more than a few parts of the buffer.

isearch.el already attempts to do that, but its method of detecting
what's out of the window doesn't work under truncate-lines.