> > Lazy highlighting searches for matches on the current window.  It must
> > be extended to search for matches on the Follow Mode group of windows.
> > For that, it needs the details of the "window*-start" and "window*-end".
>
> Yes, you're right, I see that now. This might still be solvable via
> setting isearch-search-fun-function, I'll need to think on that for a
> moment.

Here's a new version, split into 3 short patches to make it easier to
understand.

This makes 2 small changes to isearch (which I think are even
justifiable on their own, regardless of follow-mode), and defines a
new function in follow-mode which takes care of two things:

Matches are highlighted on all windows

When the user types something, and the next match is on another
window, switch to that window instead of scrolling the current one.