2011777 - Codepen demo with `anchor-center` does not center when anchor is in non-initial scroll position

Status: ASSIGNED

(Core :: CSS Parsing and Computation, defect)

Description

[David Shin[:dshin]]

(From https://github.com/w3c/csswg-drafts/issues/13353)
STR: Load demo, scroll right so the button is in view, then click on the button.
Expected: Positioned element positions on bottom center of the anchor
Actual: Positioned element positioned on bottom of the anchor, but seems to position to the right edge of screen.

Comment 1

[David Shin[:dshin]]

Attachment: Reduced testcase

We need to account for the relative movement of the anchor w.r.t. the containing block, like we do for position-area.