>> > And the patch you proposed earlier doesn't fix this other problem?
>> 
>> Unfortunately, it doesn't fix this, since 28.0.90 uses mru,
>> but 27.2 didn't.
>
> FWIW, I don't see any catastrophe here: whoever plays such games with
> switching windows during completion should know what they are doing,
> and how to switch back to the minibuffer.

Anyway, here is the patch for the release branch
that fixes the previous regression: