[css-overflow] move overscroll behaviors to overflow-4

css-overflow-4/Overview.bs

@@ -23,6 +23,7 @@ spec:css-pseudo-4; type:selector; text:::first-letter
 spec:css-pseudo-4; type:selector; text:::first-line
 spec:css-writing-modes-4; type:dfn; text:start
 spec:css-writing-modes-4; type:dfn; text:end
+spec:css2; type:dfn; text:viewport
 </pre>
 <pre class="anchors">
 url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;
@@ -1244,6 +1245,191 @@ Fragmentation of Overflow: the 'continue' property</h3>
 	This is regardless of whether this would cause any content after the [=clamp point=]
 	to be within the container's bounds.
 
+<h2 id=overscroll-behaviors>
+Overscroll Behaviors</h2>
+
+<h3 id=scroll-chaining-and-boundary-default-actions>
+Scroll chaining and boundary default actions</h3>
+
+<em>Operating Systems have rules for scrolling such as scroll chaining and
+overscroll affordances. This specification does not mandate if and how scroll
+chaining or overscroll affordances be implemented. This specification only
+allows the content author to disable them if any are implemented.</em>
+
+<dfn>Scroll chaining</dfn> is when scrolling is propagated from one
+[=scroll container=] to an ancestor [=scroll container=] following the
+[=scroll chain=]. Typically scroll chaining is performed starting at the event
+target recursing up the [=containing block chain=]. When a [=scroll container=]
+in this chain receives a scroll event or gesture it may act on it and/or pass it
+up the chain. Chaining typically occurs when the [=scrollport=] has reached its
+boundary.
+
+A <dfn>scroll chain</dfn> is the order in which scrolling is propagated from one
+[=scroll container=] to another. The [=viewport=] participates in
+[=scroll chaining=] as the document's {{Document/scrollingElement}}, both
+regarding placement in the scroll chain as well as adhering to the chaining
+rules applied to it.
+
+
+<dfn export>Scroll boundary</dfn> refers to when the scroll position of a
+[=scroll container=] reaches the edge of the [=scrollport=]. If a scroll
+container has no potential to scroll, because it does not [=overflow=] in the
+direction of the scroll, the element is always considered to be at the scroll
+boundary.
+
+<dfn>Boundary default action</dfn> refers to the user-agent-defined
+default action performed when scrolling against the edge of the [=scrollport=].
+A <dfn>local boundary default action</dfn> is a [=boundary default action=]
+which is performed on the [=scroll container=] 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.
+
+
+<h3 id=overscroll-behavior-properties>
+Overscroll Behavior Properties</h3>
+
+The <dfn>overscroll behavior</dfn> controls the permitted
+[=boundary default action=] for a [=scroll container=] element when its
+[=scrollport=] reaches the boundary of its scroll box.
+
+The 'overscroll-behavior' property specifies the [=overscroll behavior=] for a
+[=scroll container=] element. It allows the content author to specify that a
+[=scroll container=] element must prevent scroll chaining and/or overscroll
+affordances.
+
+An element that is not [=scroll container=] must accept but ignore the values of
+this property. This property must be applied to all scrolling methods supported
+by the user agent.
+
+Note: This property should provide guarantees that are, at least, as strong as
+[=preventDefault=] for preventing both scroll chaining and overscroll. Doing
+otherwise would cause content authors to use [=preventDefault=] instead.
+
+<pre class=propdef>
+Name: overscroll-behavior
+Value: [ contain | none | auto ]{1,2}
+Initial: auto auto
+Applies to: <a>scroll container</a> elements
+Inherited: no
+Media: visual
+Computed value: see individual properties
+Animation type: discrete
+Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
+</pre>
+
+
+The 'overscroll-behavior' property is a shorthand property that sets the
+specified values of 'overscroll-behavior-x' and 'overscroll-behavior-y' in that
+order. If only one value is specified, the second value defaults to the same
+value.
+
+Values have the following meanings:
+
+<dl dfn-for="overscroll-behavior, overscroll-behavior-x, overscroll-behavior-y, overscroll-behavior-inline, overscroll-behavior-block" dfn-type="value">
+	<dt><dfn>contain</dfn>
+	<dd>
+		This value indicates that the element must not perform <a>non-local boundary default actions</a>
+		such as scroll chaining or navigation. The user agent must not perform scroll chaining to any
+		ancestors along the <a>scroll chain</a> regardless of whether the scroll originated at this
+		element or one of its descendants. This value must not modify the behavior of how <a>local
+		boundary default actions</a> should behave, such as showing any overscroll affordances.
+	<dt><dfn>none</dfn>
+	<dd>
+		This value implies the same behavior as <a value for=overscroll-behavior>contain</a> and in
+		addition this element must also not perform <a>local boundary default actions</a> such as
+		showing any overscroll affordances.
+	<dt><dfn>auto</dfn>
+	<dd>
+		This value indicates that the user agent should perform the usual <a>boundary default action</a>
+		with respect to <a>scroll chaining</a>, overscroll and navigation gestures.
+</dl>
+
+Note: In the case where a user agent does not implement scroll chaining and
+overscroll affordances, these values will have no side effects for a compliant
+implementation.
+
+Note: Programmatic scrolling is clamped and can not trigger any
+[=boundary default actions=].
+
+<h3 id=overscroll-positioned>
+Overscroll and Positioned Elements</h3>
+
+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.
+
+
+<h3 id=overscroll-behavior-longhands-physical>
+Physical Longhands for 'overscroll-behavior'</h3>
+
+<pre class=propdef>
+Name: overscroll-behavior-x, overscroll-behavior-y
+Value: contain | none | auto
+Initial: auto
+Applies to: <a>scroll container</a> elements
+Inherited: no
+Logical property group: overscroll-behavior
+Percentages: N/A
+Media: visual
+Computed value: as specified
+Animation type: discrete
+Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
+</pre>
+
+The 'overscroll-behavior-x' property specifies the <a>overscroll behavior</a> in the horizontal
+axis and the 'overscroll-behavior-y' property specifies the <a>overscroll behavior</a> in the
+vertical axis. When scrolling is performed along both the horizontal and vertical axes at the
+same time, the <a>overscroll behavior</a> of each respective axis should be considered
+independently.
+
+
+<h3 id=overscroll-behavior-longhands-logical>
+Flow-relative Longhands for 'overscroll-behavior'</h3>
+
+<pre class=propdef>
+Name: overscroll-behavior-inline, overscroll-behavior-block
+Value: contain | none | auto
+Initial: auto
+Applies to: <a>scroll container</a> elements
+Inherited: no
+Logical property group: overscroll-behavior
+Percentages: N/A
+Media: visual
+Computed value: as specified
+Animation type: discrete
+Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
+</pre>
+
+These properties correspond to the 'overscroll-behavior-x' and
+'overscroll-behavior-y' properties. The mapping depends on the element's
+'writing-mode'.
+
 <h2 id=sbg-ext class=nonum>
 Appendix A: Possible extensions for ''scrollbar-gutter''</h2>