This seems to be a fairly common perspective, and I'll try to take it at face value and respond to it in parts. Sorry if I'm late and you got a bunch of other lengthy responses already.

> For example, a lot of people who have vocally criticized America in the last 5 years as being a fascist country seem to be pretty opposed to liberal values like freedom of speech and nonviolence with many such people either rationalizing left-wing violence (BLM riots as well as general antifa violence) if not outright arguing that political violence and even (capital-R) Revolution is necessary.

One problem I've had with this perspective is that the BLM movement is ongoing and has developed into a more coherent (but still decentralized) organization. According to the The Armed Conflict Location & Event Data Project (https://acleddata.com/2020/09/03/demonstrations-political-vi...):

"Between 24 May and 22 August, ACLED records more than 10,600 demonstration events across the country. Over 10,100 of these — or nearly 95% — involve peaceful protesters. Fewer than 570 — or approximately 5% — involve demonstrators engaging in violence."

I disavow the violence and rioting. The looting and destruction of storefronts is unconscionable. But it doesn't represent the majority of the movement.

As for Antifa, that ends up being a whole conversation unto itself—a poignant one given the main topic of fascism. Antifa exists as an idea more so than a formal movement. It's not something you are, it's something you do. It is a form of counter protest directed at fascists and (particularly in the US) white supremacists. It isn't a club with members that you join. The most publicized actions are violent, but more often there is no violence involved. You can counter-protest a white nationalist rally, but you can also call the hotel they are staying at and warn the management that white nationalists will be frequenting the establishment.

I'd recommend watching https://www.youtube.com/watch?v=bgwS_FMZ3nQ for a decent summary.

> Many support communism and talk about how great life was in the USSR or how amazing China is, which suggests that they're not just referring to some abstract communism that "hasn't been tried yet", but rather specific instances of communist regimes that tick many (all?) of the 'fascism qualities' boxes.

Funny enough, there's a derisive term for this in progressive circles: "Tankies"

They're an exceptionally fringe group, especially in the US, and I would appreciate some additional evidence for the claim that "many" support the USSR and China. Even for people on the left who explicitly advocate for socialism, they do so in advocacy of social justice and workplace democracy, neither of which is reflected at all in the USSR or China.

> many left-wing Americans seem to have pretty segregationist views on race even if they don't have a "master race" per se (perhaps one could argue that "people of color" is their "master race" in the way that various European identities coalesced into "white" in prior centuries?).

Again, I'd appreciate if you could provide some evidence for your claim of "many" left-wing Americans holding these views. I agree that in the last 20-30 years there has been massive re-segregation of schools in the US, and a significant demographic of Biden voters are the people who live in these very segregated suburbs. However, this issue is a common talking point for progressives in this country. I'd argue they're the only group bringing it up and attempting to address it. As for the "master race" thing, I am not entirely sure how to respond to this. Of course you can find any sort of fringe belief on the internet, but I have never seen any traction for this ideology in any of the progressive movements in the US. I know it has been a talking point when criticizing BLM (e.g. countering with "all lives matter"), but this criticism to me is intentionally misunderstanding the movement. BLM is meant to be aspirational, not exclusionary: Black lives also matter.

Two final notes:

1. You talk about a horseshoe where the fringes of the political spectrum are closer together. This is, in my opinion, an incorrect projection of the political landscape, since it implies a single dimension between right and left. I would encourage you to look at https://www.politicalcompass.org. One could get more complicated about it, but I think the distinction on authoritarian/libertarian and conservative/liberal is important. You can be a right-wing or left-wing authoritarian (e.g. Stalin vs. Hitler) and you can be a left-wing or right-wing libertarian (e.g. Noam Chomsky vs. Robert Nozick).

2. I don't want to dismiss the possibility or danger of left-wing authoritarianism. We have plenty examples of that. I do what to emphasize that fascism in particular is not one of them. It is a specific form of conservative authoritarianism that relies heavily on conservative ideology, specifically the desire for hierarchy. In every political movement we would identify as fascist, progressives and communists were the first group to be attacked as the fascists rose to power.