The term is named after American policy analyst Joseph Overton, who stated that an idea's political viability depends mainly on whether it falls within this range, rather than on politicians' individual preferences. According to Overton, the window frames the range of policies that a politician can recommend without appearing too extreme to gain or keep public office given the climate of public opinion at that time.
The window is bifurcating. The political left and right have moved so far apart that "middle ground" ideas appeal to very few and the extremes have more viability.
There basically is no political left to speak of and there hasn’t been one for decades. There is a bifurcation, but it’s largely a drift between a broadly rightward shift across political mainstreams on the one hand, and an increasingly contentious concept of who is allowed to exist and to what extent they can exist unperturbed.