But this wasn't the point. I just wanted to show that the proposed
behavior (new frame for every thread with user input) could result in
surprises.
But if the user really fires off 250 threads, then it makes sense that they get that many frames. After all, this is a computer and they are supposed to do what you tell them to do.
In any case, just magically switching focus to a y-or-n-p that was called in a different thread sounds like it could be incredibly error prone.
Imagine if you're typing in a buffer and you're writing a word that has the letter y in it. As you are about to insert the y into the buffer, a background thread asks "are you sure you want to delete all your files?".
Even if you check the minibuffer before you type every key, the thread could still steal focus just before you type a key.
I don't care much about how this is implemented, but I certainly would not want focus to be switched off without warning.
Regards,
Elias