I have been using this snippet without a problem for a while.  Is it
sufficient
to resolve the issues we are discussing with mouse-position and
mouse-pixel-position?

    (setq mouse-position-function
  (lambda (frame-x-dot-y)
    "Under macOS and Windows 7, mouse-position and mouse-pixel-position
sometimes return the prior frame; change to always return the selected
frame."
    (if (consp frame-x-dot-y) (setcar frame-x-dot-y (selected-frame)))
    frame-x-dot-y))

Bob