Since the working text is not the final text to insert to buffer, how about displaying it using after-string property of overlay? Such as:

(overlay-put (setq ns-working-overlay (make-overlay start (point)))
   ‘after-string (propertize ns-working-text ‘face 'ns-working-text-face))

在 2019年12月19日 +0800 AM8:32，tsuucat via Bug reports for GNU Emacs, the Swiss army knife of text editors <bug-gnu-emacs@gnu.org>，写道：
>
> > Can I ask what this change does?
> >
> > - (overlay-put (setq ns-working-overlay (make-overlay start (point)
> > - (current-buffer) nil t))
> > + (overlay-put (setq ns-working-overlay (make-overlay start (point)))
>
>
> At first I thought it would be fixed just by moving [self deleteWorkingText] in nsterm.m.
> However as a result insertText (ex: pressing RET to input selected candidates) left nothing.
>
> That’s because ns-delete-working-text (by [self deleteWorkingText]) delete all inserted
> chars including chars inserted by insertText-emited normal input events. This patch prevent
> ns-working-overlay expanding its size in insertText.
>
>
> Unfortunately I found the patch change behavior of undo worse. So NS-spesific patch should
> have further modification.
>
> --
> tsuucat
>
>
>
>