Thanks Jim for this higher res crop because now it's even more clear that the edge of the mirror is in front of the lighter object behind.
The mirror is obviously in front of what we see of the woman's bag! The bullet impact is in the same visual zone and it's reflections/refractions of light are sparkling out, in front of the edge of the mirror.
This (scoop) is proof of another bullet having been fired at about the same time as the neck shot. If it had been fired before that (say, a ricochet from Oswald's missed shot?) the agents in the front of the car would have reacted right away and gone into flee mode.