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