In the attached test I have a transparent Frame whose opacity is bound to a slider in such way that the max slider value at 255 corresponds to a opacity value of 1.
At slider values 12 and below, corresponding to a fractional opacity value of 12/255 and below, the transparent Frame is just barely visible, the events are picked by the native system - the mouse entered, mouse exited and mouse moved events are caught by the FX frame. However, if you try to mouse press and drag over the red rectangle, the event is picked up by the native system.