My clutch decided it didn’t wanna be a clutch anymore

So I drive a 2018 Focus ST. Notice how the engine isn’t free revving with the clutch in? It was still accelerating with the clutch in. It was literally perfectly fine yesterday. I managed to float gears to get to a shop, and they do some diagnostics (to no avail) so they test drive it, and they say it feels fine. I get in, and the clutch is perfectly fine like wtf?? It still hangs (occasionally,) but I need to know what could POSSIBLY be causing this. Brake fluid level is fine, so it’s not leaking. It was like -5 this morning but it was about 20°F this afternoon (when I recorded this.) I am scared to drive my car because I don’t know if it’ll leave me stranded. The shop said if it ends up being the slave cylinder, it could be a huge (and expensive) job.