isChildWindowKey = key => isString(key) /* skipping Symbol as it throws on conversion */
  && (key = +key) >= 0 /* is number */
  && (key === (key | 0)) /* is 32-bit integer, no fraction */
  && key < window::getWindowLength()
Original ~50 global functions such as setTimeout that some sites override.
Not saving all globals because it would waste a lot of time on each page and frame.