Handling of [mouse-movement] and exiting isearch

[Daniel Martín]

I'd like commands bound to [mouse-movement] to not exit isearch, and I'm
confused by this code in isearch.el:

    ;; Pass frame events transparently so they won't exit the search.
    ;; In particular, if we have more than one display open, then a
    ;; switch-frame might be generated by someone typing at another keyboard.
    (define-key map [switch-frame] nil)
    (define-key map [delete-frame] nil)
    (define-key map [iconify-frame] nil)
    (define-key map [make-frame-visible] nil)
    (define-key map [mouse-movement] nil)
    (define-key map [language-change] nil)

The comment seems to imply that [mouse-movement], [language-change] and
some frame-related events won't exit isearch.  However, the code is
calling define-key and passsing KEY as nil, which means that the events
will be undefined in isearch-mode-map, which by default means that they
_will_ exit isearch, right? I think the code that implements this logic
is in function isearch-pre-command-hook:

   ;; Don't exit Isearch for isearch key bindings.
   ((or (commandp (lookup-key isearch-mode-map key nil))
        (commandp
         (lookup-key
          `(keymap (tool-bar menu-item nil ,isearch-tool-bar-map))
   key))))

What am I missing? I'm not sure if making [mouse-movement] not exit
isearch worked in the past but has regressed in recent versions of
Emacs, or if I'm not understanding the logic correctly.

Thanks.