Re: completing-read (and M-x) with pop-up-frames non-nil changesframefocus

[Richard M. Stallman]

        When focus follows the mouse, it is difficult to avoid selecting it
        if it shows up under the mouse.

    I didn't have focus-follows-mouse turned on.

    If it were on, and if the new frame did not happen to show up under the
    mouse, it would not be selected, right?

No, that variable is meant to tell Emacs what the window manager does.

    I guess you're saying that when a frame is created it is always given the
    focus. That's too bad.

I am saying that the system decides what frame gets focus.