I've attached a draft implementation of a minibuffer-controlled mode for
Isearch, as described for instance in
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00447.html
To enable the feature, set `isearch-from-minibuffer' to t.

The basic trade-off is that this makes it easier to edit the search
string, and harder to quit the search.  It also drops some of the
eccentricities of regular Isearch.  For instance, DEL deletes and C-g
quits.

I'm sharing this preliminary version because two important questions can
already be answered:

- Does the approach taken here seem sufficiently robust?  Note in
  particular the `with-isearch-window' macro, which is now needed around
  several functions, as well as the somewhat hacky `run-with-idle-timer'
  call inside the `isearch-mode' function.

- Are the slightly backwards incompatible keybinding changes in
  `isearch-edit-string' acceptable?

If any of these answers is no, then I would provide a package for the
same feature.  But I think the feature is interesting enough to be built
in isearch.el.  Moreover, it would benefit from being official because
many third-party extensions to Isearch will need to take into account
the possibility that the search is being controlled remotely from a
minibuffer.

Some further remarks:

- The minibuffer-controlled mode is supposed to depend on the proposed
  `isearch-buffer-local' feature.  This will make the hack used to
  deactivate the `overriding-terminal-local-map' unnecessary.

- It seems necessary to let-bind `inhibit-redisplay' to nil in
  `with-isearch-window' in order to avoid flicker in the cursor.  This
  seems related to the recent thread "Temporarily select-window, without
  updating mode-line face and cursor fill?" in this list.  Any better
  solutions?

- I don't like the `with-isearch-window-quitting-edit' macro, but I
  don't see a different way of achieving the necessary effect.

- I don't use/know of all Isearch features, so let me know if you spot
  some incompatibility.

What do you think?