I don’t think the average brain works like this. If you know you have a piece of code to deliver or are in flow, I don’t think you’d willingly start browsing HN just because you went to SO to c&p some boilerplate code.

I find that if my brain is willing to be distracted, it’s made some sort of calculation saying that the cost of being distracted isn’t going to have a significant bearing on deliverables…and I’m pretty sure I’m no better or worse at estimations than anyone else.