The rest of this is a re-post of a slightly edited version of a post I made over there.
This all started when I checked CricViz' calculation of 'who was winning', against my own rather more nebulous 'Test Win Expectancy' model during the Mendis-Matthews partnership. Going by the match position, I thought Sri Lanka had grounds for optimism, even to the point that I suspected CricViz was doing something badly wrong. Having studied the data a bit more thoroughly in the light of subsequent events, I have to set my optimism for Sri Lanka's chances aside altogether. Basically, I had overrated the batting resources remaining to Sri Lanka. The actual target is not unattainable, on the face of it, but one wouldn't want to start from here.
In bald terms, Sri Lanka need two century partnerships between Mendis, Dickwella and a returned Chandimal. (A 50 from Dilruwan Perera would reduced the pressure on those three, but we're still talking about three big partnerships.)
The wickets of Kusal Perera and Silva fell at scores below par, while the Mendis-Matthews partnership was well above par. However, given the mammoth number of the target, Sri Lanka need multiple wickets to go well above par. Basically Mendis has already done his bit, and it is expecting too much of him to add substantially to his current score. (It could happen, of course, to the delight of Sri Lanka's supporters.)
So, now we are down to an uncertain Chandimal and Dickwella, who has a Test average of 33 and no centuries to his name. With those resources, it's hard to feel any optimism that 200 runs could be scored over the next two wickets. Of course, tails can wag, but the Sri Lankan tail-enders will be hard-pressed to make 77 runs between them, going by their averages, which make James Anderson look good.
As I suspected, and one can read on Reddit, Silva's wicket was crucial. My mistake was not to weight it heavily enough in subsequent posts. CricViz' 89% chance of a WIndies' victory doesn't look so far-fetched.