Fixed position is explicitly called out in the W3C visual formatting model editor's draft as establishing a stacking context:
https://drafts.csswg.org/css2/visuren.html#z-index

Inadvertently we ended up considering position: sticky the same as position fixed when stuck to the viewport, but unless it also has a z-index specified it seems that it should not create a stacking context.

See http://output.jsbin.com/yucorey where the green box should be on top of the red box.