[css-position-3] Deal with overflowing content as well.

Author: fantasai

css-position-3/Overview.bs

@@ -595,9 +595,13 @@ Sticky positioning</h3>
 	the corresponding edge of the [=sticky viewport rectangle=],
 	then the box must be visually shifted (as for relative positioning)
 	to be inward of that [=sticky viewport rectangle=] edge,
-	insofar as it can while its [=margin box=]
-	(less any margins that have collapsed with its parent)
+	insofar as it can while its [=position box=]
 	remains contained within its [=containing block=].
+	The <dfn>position box</dfn> is its [=margin box=],
+	except that for any side for which the distance between its [=margin edge=]
+	and the corresponding edge of its [=containing block=]
+	is less than its corresponding [=margin=],
+	that distance is used in place of that [=margin=].
 
 	Note: A sticky positioned element with
 	a non-''auto'' 'top' value and an ''auto'' 'bottom' value
@@ -608,8 +612,6 @@ Sticky positioning</h3>
 	are offset independently,
 	and therefore might overlap.
 
-	Issue: Say what happens if it already overflows the containing block
-
 <!-- End section: Sticky positioning	-->
 
 <h3 id="abs-pos">