>
> I get much better scrolling experience with these properties:
>
> (put 'scroll-up-command 'isearch-motion
>     '((lambda () (goto-char (window-end)) (recenter 1 t)) . forward))
> (put 'scroll-down-command 'isearch-motion
>     '((lambda () (goto-char (window-start)) (recenter -1 t)) . backward))
>
> so only new unseen matches are highlighted.
>

Indeed, I tried them, and I agree with you that these properties give a 
better experience.

>> +        (when (and isearch-success (not isearch-motion-changes-direction))
>> +          (unless (eq direction current-direction)
>> +            (let ((isearch-repeat-on-direction-change nil))
>> +              (isearch-repeat current-direction))))
>
> This 'isearch-repeat' is broken when the new option 
> 'isearch-repeat-on-direction-change' is customized to t.
>

Fixed.

>
> Also in this case the old commands isearch-beginning-of-buffer and 
> isearch-end-of-buffer are broken too.  Maybe you need to wrap their 
> calls of 'isearch-repeat' with
>
>    (let ((isearch-repeat-on-direction-change nil))
>      (isearch-repeat 'forward arg))
>

Indeed, it is necessary to let-bind isearch-repeat-on-direction-change to 
nil there, like elsewhere.  I attach a separate patch, because it's not 
related to the isearch-allow-motion patch.