Spent a few hours trying to reproduce the issue and testing various cases. It seems that it happens pretty rarely when attempting to repro.
I think it only happens for specific timing situations - in my test, I always type a letter early in the document which changes the parsing context, then scroll down to the bottom. Depending on the timing, most of the time, parsing catches up as I scroll (which seems to be the intended behavior). Only in rare occasions, the parsing logic seems to be completely stopped until a blur & re-focus happens. It also doesn’t seem to happen as much when scrolling using the scroll wheel, compared to scrolling by dragging the scrollbar.
It’s probably some kind of race condition, but I’m at a loss for more details. I’ve repro’d on a ~16kb and another ~80kb document.