>> Do you think this problem exists only
>> when isearch-lazy-count is customised to t?
>
> No, I can just launch ‘emacs -Q’, go to the beginning of the buffer
> and start searching for something with ‘C-s M-s '’ to get the
> flickering prompt. What the settings in the first message do is change
> the prompt: it is ‘Pending char-fold I-search’ without them (after
> doing ‘C-s M-s '’) and just ‘I-search:’ with them (where just ‘C-s’
> suffices for starting a char-folded search).
>
> Your patch works both with and without my settings. I don’t know if
> there are cases where it suppresses useful messages.

A possible variant would be not to flicker too much by
transitioning from the message:

  Pending char-fold I-search: string

to

  1/20 Pending char-fold I-search: string

whereas currently it changes to

  1/20 Char-fold I-search: string

by removing the prefix "Pending" after a delay.

But maybe better not to display "Pending" at all?

So with the following patch, adding or removing characters in the search
string doesn't show with the message "Pending" in char-fold mode: