Created attachment 478269 [details] Minimum Reproducible Case (Discovered from Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=2014997) Given that: 1. The initial fixed containing block is the parent of the initial containing block in the containing block chain [1], 2. Absolute containing block can have at most the initial containing block as its absolute containing block, 3. And an acceptable anchor should be strictly laid out before the positioned element [2], Strictly spec-wise, fixed elements (Using the initial fixed containing block) should not be a valid anchor for an absolute positioned element in the testcase. Perhaps relatedly - the positioned element doesn't seem to be properly scroll (un?)compensated. [1] https://drafts.csswg.org/css-position-3/#def-cb [2] https://drafts.csswg.org/css-anchor-position-1/#acceptable-anchor-element