Overview: We aren't applying scroll adjustments to position:fixed boxes, so they are separating from their anchors when the page is scrolled. Affected WPT Tests: * http://wpt.live/css/css-anchor-position/anchor-scroll-fixedpos.html This is a basic test for the functionality. * http://wpt.live/css/css-anchor-position/anchor-scroll-006.html This checks tracking for anchors in descendant scrollers. * http://wpt.live/css/css-anchor-position/anchor-scroll-007.html This checks tracking when multiple anchors are involved. * http://wpt.live/css/css-anchor-position/position-try-switch-to-fixed-anchor.html This checks the behavior when applied via position-try. Missing Tests: * Need to add a test for `position-area`. * Need to add a test to check that position-try fallbacks are triggered.