It's mostly a categorical problem - it's a hard term to pin down. I've been thinking for the past few years that late 60s/early 70s prog-rock played a much more substantial role in it then I had initially assumed. Take say Tocatta from ELP's BSS album: https://youtu.be/Qk8jAuVzga8?t=494 ... that's 1973 - a careful ear will recognize many later standard patterns packed in that small clip. There's tons of these little 20-second or so clips that are 20 years ahead of their time in the prog-rock from the era.

Along with that are space age and lounge jazz. Take Hugo Montenegro – Moog Power from 1969 for instance or the Mort Garson album under the name Atraxia from 1975.

I used to think these weren't part of the story but I've been coming around on it