This. To elaborate, the problem is that without the "window shopping" optimisation, you are forced to repeat all your "window shopping" excursions twice every time you want to go a little further back in time.
To illustrate how horrible this is, I'll continue the example: When you paste that something, you get state4. Suppose you then wanted to copy something from state1. Without the "window shopping" optimisation, you would have to go state4,state3,state2,state3,state2,state1, copy, state2,state3,state2,state3,state4, paste.
If you then wanted to copy something from state0, it would take 34 steps!
To illustrate how horrible this is, I'll continue the example: When you paste that something, you get state4. Suppose you then wanted to copy something from state1. Without the "window shopping" optimisation, you would have to go state4,state3,state2,state3,state2,state1, copy, state2,state3,state2,state3,state4, paste.
If you then wanted to copy something from state0, it would take 34 steps!