[css-position-3] Define that stickypos elements are at their initial …

…position when a scrolling operation tries to target them. #1459

css-position-3/Overview.bs

@@ -637,6 +637,22 @@ Sticky positioning</h3>
 	are offset independently,
 	and therefore might overlap.
 
+<h4 id="stickypos-scroll">
+Scroll Position of Sticky-Positioned Boxes</h4>
+
+	For the purposes of any operation targetting the scroll position
+	of a sticky positioned element (or one of its descendants),
+	the sticky positioned element must be considered to be positioned
+	at its initial (non-offsetted) position.
+
+	<div class="example">
+		For example, if a user clicks a link targetting a [=sticky-positioned=] element,
+		even if the element's [=nearest scrollport=] is currently scrolled
+		such that the [=sticky positioned=] element is offset from its initial position,
+		the [=scrollport=] will be scrolled back 
+		so that the element's initial position is visible.
+	</div>
+
 
 <h3 id="abspos-insets" oldids="abs-pos">
 Absolute Positioning</h3>