<pre class='metadata'>
Title: CSS Overscroll Behavior Module Level 1
Shortname: css-overscroll
Level: 1
Status: ED
Work Status: Exploring
Group: CSSWG
URL: https://drafts.csswg.org/css-overscroll-1/
TR: https://www.w3.org/TR/css-overscroll-1/
Former Editor: Benoit Girard, Facebook, bgirard@fb.com
Former Editor: Majid Valipour, Google, majidvp@google.com, w3cid 81464

Abstract: This module defines 'overscroll-behavior' to control the behavior when the scroll position
Abstract: of a <a>scroll container</a> reaches the edge of the <a>scrollport</a>.
Abstract: This allows content authors to hint that the <a>boundary default actions</a>,
Abstract: such as scroll chaining and overscroll, should not be triggered.
</pre>


Introduction {#intro}
=====================

<em>This section is not normative.</em>

A content author does not necessarily want <a>scroll chaining</a> to occur for all <a>scroll
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 {{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 {{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 {{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. {{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
robust, performant and forward compatible way. The 'overscroll-behavior' property fixes this
shortcoming.

Value Definitions {#values}
---------------------------

This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.

Motivating Examples {#motivating-examples}
==========================================


<div class=example>
A position fixed left navigation bar does not want to hand off scrolling to the document because a
scroll gesture performed on the navigation bar is almost never meant to scroll the document. Note
that using the native overscroll affordances are still desirable while scroll chaining is to be
prevented.

  <pre class="lang-css">
  #sidebar {
    overscroll-behavior: contain;
  }
  </pre>

In this case, the author can use <a value for=overscroll-behavior>contain</a> on the sidebar to
prevent scrolling from being chained to the parent document element.
</div>

<div class=example>
A page wants to implement their own pull-to-refresh effect and thus needs to disable browser
native overscroll action.

  <pre class="lang-css">
  html {
    /* only disable pull-to-refresh but allow swipe navigations */
    overscroll-behavior-y: contain;
  }
  </pre>

In this case, the author can use <a value for=overscroll-behavior>contain</a> on the viewport
defining element to prevent overscroll from triggering navigation actions.
</div>

<div class=example>
A infinite scrollers loads more content as user reaches the boundary and thus wants to disable the
potentially confusing rubber banding effect in addition to scroll chaining.

  <pre class="lang-css">
  #infinite_scroller {
    overscroll-behavior-y: none;
  }
  </pre>

In this case the author can use <a value for=overscroll-behavior>none</a> on the infinite
scroller to prevent both scroll chaining and overscroll affordance.
</div>

Scroll chaining and boundary default actions {#scroll-chaining-and-boundary-default-actions}
============================================================================================

<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 <a>scroll container</a>
to an ancestor <a>scroll container</a>
following the <a>scroll chain</a>.
Typically scroll chaining is performed starting at the event target
recursing up the <a>containing block chain</a>.
When a <a>scroll container</a> 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 <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.
For a given [=scroll container=] and scroll,
the next link in its [=scroll chain=]
is the nearest ancestor [=scroll container=]
in the [=containing block chain=]
that is scrollable in the scroll's axis.
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 not <a>overflow</a> in the direction of the scroll,
the element is always considered to be at the scroll boundary.
A [=single-axis scroll container=] only has a [=scroll boundary=] in its [=scrollable axis=];
if the scroll is in its other axis,
this element won't be part of the [=scroll chain=] at all.

<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.



Overscroll Behavior Properties {#overscroll-behavior-properties}
================================================================

The <dfn>overscroll behavior</dfn> controls the permitted <a>boundary default action</a> for a
<a>scroll container</a> element when its <a>scrollport</a> reaches the boundary of its scroll box.


The 'overscroll-behavior' property specifies the <a>overscroll behavior</a> for a <a>scroll
container</a> element. It allows the content author to specify that a <a>scroll container</a>
element must prevent scroll chaining and/or overscroll affordances.

An element that is not <a>scroll container</a> 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 <a>preventDefault</a>
for preventing both scroll chaining and overscroll. Doing otherwise would cause content authors to
use <a>preventDefault</a> instead.


The 'overscroll-behavior' Shorthand {#overscroll-shorthand}
-----------------------------------

<pre class=propdef>
Name: overscroll-behavior
Value: [ contain | none | auto | chain ]{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>chain</dfn>
  <dd>
    This value indicates that the element may perform <a>non-local boundary default actions</a>
    such as scroll chaining or navigation. However, the element must 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 <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}
---------------------------------------------------------------------------------------

<pre class=propdef>
Name: overscroll-behavior-x, overscroll-behavior-y
Value: contain | none | auto | chain
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.


Flow-relative Longhands for 'overscroll-behavior'  {#overscroll-behavior-longhands-logical}
-------------------------------------------------------------------------------------------

<pre class=propdef>
Name: overscroll-behavior-inline, overscroll-behavior-block
Value: contain | none | auto | chain
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'.


Security Considerations {#security}
====================================
There are no known security impacts of this feature. The feature may be used to prevent
certain native UI features such as overscroll affordances and overscroll navigations (e.g., pull-
to-refresh, swipe navigations). However, this does not expose any additional abilities beyond what
is already possible in the platform e.g., by preventing the default action of the event that would
cause a scroll.

Privacy Considerations {#privacy}
=================================
No new privacy considerations have been reported on this specification.
