Right now the flex highlighter tool highlights children that are absolutely positioned & fixed-positioned. However, these children are not flex items.[1] So we probably shouldn't highlight them -- or we should highlight them differently than the "real" flex items. (These elements are still a little bit special, because their static position [i.e. the position if "top"/"left"/"right"/"bottom" is unset] *is* determined using CSS Alignment & the size of the flex container[2] -- kind of as if they were the only flex item in the container. But they don't flex at all, and they don't interact with the "real" flex items.) [1] https://drafts.csswg.org/css-flexbox-1/#abspos-items [2] https://drafts.csswg.org/css-align/#align-abspos-static

Actually, looks like this will be fixed in bug 1435749 (when we switch to using the API that was added in bug 1409083). I'll just dupe this to bug 1435749.