[css-overscroll-behavior-1] Clarify viewport participation in scroll chaining #3267

Author: majido

css-overscroll-behavior-1/Overview.bs

@@ -21,6 +21,10 @@ urlPrefix: https://www.w3.org/TR/css-display-3/
 url: https://drafts.csswg.org/css-overflow-3/#scroll-container
     type: dfn; text: scroll container
     type: dfn; text: scroll containers
+url: https://drafts.csswg.org/cssom-view/#viewport
+    type: dfn; text: viewport
+url: https://drafts.csswg.org/cssom-view/#dom-document-scrollingelement
+    type: dfn; text: scrollingElement
 url: https://www.w3.org/TR/uievents/#default-action
     type: dfn; text: default action
     type: dfn; text: default actions
@@ -125,7 +129,10 @@ performed starting at the event target recursing up the <a>containing block chai
 pass it up the chain. Chaining typically occurs when the <a>scrollport</a> has reached its boundary.
 
 A <dfn>scroll chain</dfn> is the order in which scrolling is propagated from one <a>scroll
-container</a> to another.
+container</a> to another. The <a>viewport</a> participates in <a>scroll chaining</a> as the
+document's <a>scrollingElement</a>, both regarding placement in the scroll chain as well as adhering
+to the chaining rules applied to it.
+
 
 <dfn>Scroll boundary</dfn> refers to when the scroll position of a <a>scroll container</a> reaches
 the edge of the <a>scrollport</a>. If a scroll container has no potential to scroll, because it does
@@ -135,9 +142,10 @@ scroll boundary.
 <dfn>Boundary default action</dfn> refers to the user-agent-defined <a>default action</a> performed
 when scrolling against the edge of the <a>scrollport</a>. A <dfn>local boundary default action</dfn>
 is a <a>boundary default action</a> which is performed on the <a>scroll container</a> without
-interacting with the page, for example displaying a overscroll UI affordance. Conversely, a <dfn
->non-local boundary default action</dfn> interacts with the page, for example scroll chaining or a
-navigation action.
+interacting with the page, for example displaying a overscroll UI affordance. Conversely, a
+<dfn>non-local boundary default action</dfn> interacts with the page, for example scroll chaining or
+a navigation action.
+
 
 
 Overscroll Behavior Properties {#overscroll-behavior-properties}