Questions about isearch

[Eli Zaretskii <eliz@gnu.org>]

These questions came out of review and extensive updates of the search
and replace sections of the Emacs manual:

1. Character folding doesn't catch ligatures, such as æ (should it match
the two characters "ae")?

2. It also doesn't match ä (a single character) with ä (2 characters,
which Emacs correctly composes into 1 grapheme cluster).  Should it?

3. With the default value t of isearch-hide-immediately, one match in
invisible text is not hidden, and remains on display.  To repro:

  emacs -Q
  C-x C-f etc/NEWS RET
  C-c C-q
  C-s require C-s <RIGHT>

This leaves the match and its surrounding hidden text on screen.  I
can understand the rationale, but the doc string doesn't say anything
about this feature.  On the contrary, it says:

  Whatever the value, all opened invisible text is hidden again after
  exiting the search. ^^^

4. What is the equivalent of case-replace and the letter-case related
behavior of replace commands to character folding?  E.g., if the
replace command specifies to replace "foo" with "bar", and we found
"föo", should we replace it with "bär" or something, by analogy with
letter-case behavior?