A year after posting that suggestion, I actually did test it for myself. And I was STUNNED to discover that that VPS optical sensor actually DOES take priority over GPS!
I was totally convinced that, given good GPS reception, the Mini would always hold position in a hands-free hover and would NOT drift away following floating leaves, or ripples on the water's surface, or current in a river. I thought it was completely illogical to believe the VPS optical sensor would track moving objects and cause the Mini to follow them while the drone had good GPS reception. I was adamant that this could only happen in the event of insufficient GPS reception.
Well. I was totally wrong!
Note: This can't happen while you are in control and actively moving the sticks. It only happens during sticks-centred hover, while you're expecting the drone to automatically hold itself in one spot. As shown in this video, the VPS optical sensor
may cause the drone to drift away if it's tracking some moving pattern below it, even
despite the drone having a solid GPS lock at the time! If you need to look away, making adjustments on screen or whatever, always keep one eye on your drone to make sure it's still where you think it should be!