Hi Dan, thanks for sharing! Indeed, visually stuff starts to flicker once you are progressing on the Path. It is one of those things that points to the fact that nothing is solid, nothing has inherent existince, that everything is "empty". It is one of those things that when you notice it the first few times, can be really creepy, then you get used to it and it actually becomes very beatufil and exciting in a "Oh woow" kind of way.
(Spoiler alert) And then at some point you realize that it has always been like that, you just didn't notice (End of Spoiler)
If you look very carefully, it is the same across all sense doors - sounds also flicker, they are these little tiny vibrations that break down into even tinier flickering vibrations you cannot actually find as inherently existing and stable. Body sensations and emotions flicker, etc. "Nothing lasts even a mili-second" as Daniel Ingram says.
I am not an expert on the detailed phenomenology of Progress of Insight, or on Buddhist technical dharma and Pali canon, so can't help too much there. It was heavily the case for me during A&P, and also sometimes during DN and EQ. I can't remember very accurately, but I believe I had it during my fist A&P and DN/EQ as well, and of course it comes into play even more heavily on subsequent paths as well. From one point onwards it is "always there" if you decide to use that on demand "way of looking" - if you just incline in the direction of seeing everything as flickering, boom - it is flickering. And if you decide to zoom-out of that way of seeing, e.g. when you drive a car or whatever, it is not so much in the foreground and things look more "regular" but still ephemeral/empty.
