"[J]ournalists weren't consciously deciding the equilibrium. The journalists were writing 'serious' articles, i.e. articles about Alice and Bob rather than Carol. The equilibrium consisted of the journalists writing sports coverage of elections, where everything is viewed through the lens of a zero-sum competition for votes between Alice's team and Bob's team. Viewed through that lens, the journalists thought a gay marriage endorsement would be a blunder. And if you do something that enough people think is a political blunder, it is a political blunder. The journalists' sports coverage will describe you as an incompetent politician and primates instinctively want to ally with like winners. Which meant the equilibrium could have a sharp tip over point, without most of the actual population changing their minds sharply about gay marriage in that particular year. The support level went over a threshold where somebody tested the waters and got away it, and journalists began to suspect it wasn't a political blunder to support gay marriage, which let more politicians speak and get away with it, and then the change of belief about what was inside the Overton window snowballed."I think this process is pretty important currently with respect to Labour. I think a lot of the commentariat believed that Labour explicitly articulating more left-wing economic ideas was A Political Blunder, and because they think that it became A Political Blunder with very little reference to the views of the public.
Then came the exit poll on 8 June 2017, and everything changed. It turns out you can advocate a more left-wing economic programme and not get demolished in an election. In fact it turns out that a lot of the public - including Conservative-voting members of the public - like these policies. I doubt the public's views actually changed, but the commentariat's views of the public's views did change. Now articulating those views is not seen as A Political Blunder, in and of itself (though the commentariat by and large hasn't moved to embrace them). Or, in the language of the excerpt, views about what is inside the Overton window have changed.
Things have obviously moved on in the past year. Some political commentators seem to want to go back to pre-8 June 2017. But to publicly advocate, say, public ownership of utilities is no longer A Political Blunder. Indeed, to defend the current system might be seen by some as more politically risky.
This does make me think a lot about Expert Political Judgement (something I have personally lacked over the past 5 years!) and the idea that we ought to rate members on the commentariat on their track record of political predictions.