[css-position-3] Clarify visual first/last fragment used. #7917

Author: tabatkins

css-position-3/Overview.bs

@@ -324,6 +324,10 @@ Containing Blocks of Positioned Boxes</h3>
 					the [=containing block=] is formed by
 					the [=block-start=] and [=inline-start=] [=content edges=] of the first [=box fragment=] of the ancestor,
 					and the [=block-end=] and [=inline-end=] [=content edges=] of the last [=box fragment=] of the ancestor.
+					If there are multiple fragments on the same line
+					(e.g. due to [[#bidi-box-model|bidi reordering]]),
+					take the [=start=]most/[=end=]most fragment
+					as the first/last fragment.
 
 					Note: If the ancestor breaks across a line,
 					the “start” position might more end-ward than the “end” position.