> In any case, yes, your suggestion of first doing what we do now
> (highlight the immediate area, using the current algorith), and
> then following that with highlighting the rest of the buffer,
> could be a good idea. Dunno how much that might change
> the existing code.

In the patch attached below, a new function isearch-lazy-highlight-buffer-update
is a copy of isearch-lazy-highlight-update with some changes specific
to highlighting the full buffer.  It seems making a duplicate function
is necessary because adding more full-buffer specific conditions to
the existing function isearch-lazy-highlight-update will make it
unmaintainable.

Only a part of the function (that highlights the match) is extracted
into a new function isearch-lazy-highlight-match and shared among these
aforementioned two functions.