I'm not sure I've ever seen one that lets you sign before the cashier is done scanning. Wouldn't that defeat the whole premise of signing "I agree to pay the amount above in full"?
You can do everything up to that point - 'what type of card is this', etc... And many places are going signature-free below a certain transaction size, so I suppose all interactions can be handled in those cases before the 'total' key is hit.
In my experience, you can get as far as needing to hit "I agree to pay" and scribbling on the touch screen.
Entering in/scanning your customer loyalty number and pulling out your credit card and scanning it (and pressing credit/debit) is what I've been able to "preload".
One other neat thing I've noticed is that at my Costco gas station, I can pull the pump out and select my gas type while simultaneously scanning my costco + credit card. I usually try to time myself with the person in front of/behind me to see if I can start my pump faster than them. I usually win :)
You can do everything up to that point - 'what type of card is this', etc... And many places are going signature-free below a certain transaction size, so I suppose all interactions can be handled in those cases before the 'total' key is hit.