[css-overscroll-1] Per WG resolution, fixpos and viewport-stuck stickypos don't overscroll. #6299

Author: tabatkins

css-overscroll-1/Overview.bs

@@ -16,25 +16,6 @@ Abstract: This allows content authors to hint that the <a>boundary default actio
 Abstract: such as scroll chaining and overscroll, should not be triggered.
 </pre>
 
-<pre class='anchors'>
-urlPrefix: https://www.w3.org/TR/css-display-3/
-    type: dfn; text: containing block chain
-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
-url: https://dom.spec.whatwg.org/#dom-event-preventdefault
-    type: dfn; text: preventDefault
-url: https://dom.spec.whatwg.org/#dom-eventtarget-addeventlistener
-    type: dfn; text: passive flag
-
-</pre>
 
 Introduction {#intro}
 =====================
@@ -45,20 +26,20 @@ A content author does not necessarily want <a>scroll chaining</a> to occur for a
 containers</a>. Some <a>scroll containers</a> may be part of a <a>containing block chain</a> but may
 serve a different logical purpose in the document and may want to prevent scrolling from continuing
 up the <a>scroll chain</a>. To achieve this, a content author will install event listeners without
-the <a>passive flag</a> set and will use <a>preventDefault</a> when there is a risk that scroll
+the <a>passive flag</a> set and will use {{Event/preventDefault()}} when there is a risk that scroll
 chaining will occur. This is detrimental for the following reasons:
 
 * The user agent may in the future introduce new input methods for scrolling that are not supported
     by the content author's event listeners.
 * A non passive event listener will delay scrolling because the user agent will have to wait for the
-    result of the event listener to determine if <a>preventDefault</a> was called causing increased
+    result of the event listener to determine if {{Event/preventDefault()}} was called causing increased
     scroll latency.
 * When scrolling is performed near the edge of the <a>scroll boundary</a>, the <a>default action</a>
     may cause both scrolling to the edge of the <a>scroll container</a> and a <a>boundary default
-    action</a>. Calling <a>preventDefault</a> will not only cancel the <a>boundary default action</a>
+    action</a>. Calling {{Event/preventDefault()}} will not only cancel the <a>boundary default action</a>
     but also the scroll to the edge of the <a>scrollport</a>.
 * The <a>default action</a> for the event may also provide additional behavior that the author does
-    not want to cancel such as an overscroll affordance. <a>preventDefault</a> doesn't allow the
+    not want to cancel such as an overscroll affordance. {{Event/preventDefault()}} doesn't allow the
     content author to cancel only some of the <a>default actions</a> such as scroll chaining.
 
 Thus, it is not possible for a content author to control <a>scroll chaining</a> and overscroll in a
@@ -222,6 +203,47 @@ these values will have no side effects for a compliant implementation.
 
 Note: Programmatic scrolling is clamped and can not trigger any <a>boundary default actions</a>.
 
+Overscroll and Positioned Elements {#overscroll-positioned}
+----------------------------------
+
+This specification does not generally dictate what, if any,
+"overscroll" or similar actions
+might occur as a [=local boundary default action=].
+
+However, if a user agent <em>does</em> use "overscroll" behavior
+(that is, allowing a scrollable element
+to be scrolled slightly "past the end" of its scrollable area,
+usually with a "rubber-banding" effect after the scroll or drag is completed),
+then the following applies:
+
+If an element uses [=fixed positioning=]
+and is positioned relative to the [=initial containing block=],
+or is a [=sticky positioned=] element
+which is currently stuck to the [=viewport=],
+then when the [=root scroller=] experiences "overscroll",
+that element <em>must not</em> overscroll with the rest of the document's content;
+it must instead remain positioned as if the scroller was at its minimum/maximum scroll position,
+whichever it will return to when the overscroll is finished.
+
+Even tho this can visually shift the fixed/sticky element
+relative to other elements on the page,
+it must be treated purely as a visual effect,
+and not reported as an actual layout/position change
+to APIs such as {{Element/getBoundingClientRect()}}.
+
+Note: This behavior is because fixpos and viewport-stuck stickypos elements
+are positioned relative to "the viewport",
+which is conceptually above the root scroller
+in the page hierarchy
+(effectively, it's the [=scroll container=]
+holding the root scroller).
+Thus, overscrolling the root scroller shouldn't have any effect on them,
+just like how an abspos
+that is a child of a scroller
+but whose abspos containing block is <em>above</em> the scroller
+isn't affected by the scroller doing anything at all,
+including overscroll.
+
 
 Physical Longhands for 'overscroll-behavior'  {#overscroll-behavior-longhands-physical}
 ---------------------------------------------------------------------------------------