CSS Overscroll Behavior Module Level 1

Editor’s Draft,

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https://drafts.csswg.org/css-overscroll-behavior-1/
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Abstract

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

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

GitHub Issues are preferred for discussion of this specification. When filing an issue, please put the text “css-overscroll-behavior” in the title, preferably like this: “[css-overscroll-behavior] …summary of comment…”. All issues and comments are archived, and there is also a historical archive.

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

1. Introduction

This section is not normative.

A content author does not necessarily want scroll chaining to occur for all scroll containers. Some scroll containers may be part of a containing block chain but may serve a different logical purpose in the document and may want to prevent scrolling from continuing up the scroll chain. To achieve this, a content author will install event listeners without the passive flag set and will use preventDefault when there is a risk that scroll chaining will occur. This is detrimental for the following reasons:

Thus, it is not possible for a content author to control scroll chaining and overscroll in a robust, performant and forward compatible way. The overscroll-behavior property fixes this shortcoming.

2. Motivating Examples

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.
#sidebar {
  overscroll-behavior: contain;
}

In this case, the author can use contain on the sidebar to prevent scrolling from being chained to the parent document element.

A page wants to implement their own pull-to-refresh effect and thus needs to disable browser native overscroll action.
html {
  /* only disable pull-to-refresh but allow swipe navigations */
  overscroll-behavior-y: contain;
}

In this case, the author can use contain on the viewport defining element to prevent overscroll from triggering navigation actions.

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.
#infinite_scroller {
  overscroll-behavior-y: none;
}

In this case the the author can use none on the infinite scroller to prevent both scroll chaining and overscroll affordance.

3. Scroll chaining and boundary default actions

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.

Scroll chaining 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 scroll chain is the order in which scrolling is propagated from one scroll container to another.

Scroll boundary 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.

Boundary default action refers to the user-agent-defined default action performed when scrolling against the edge of the scrollport. A local boundary default action 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

non-local boundary default action interacts with the page, for example scroll chaining or a navigation action.

4. Overview

This module introduces control over the behavior of a scroll container element when its scrollport reaches the boundary of its scroll box. It allows the content author to specify that a scroll container element must prevent scroll chaining and/or overscroll affordances.

5. Overscroll Behavior Properties

These properties specify how a scroll container element must behave when scrolling. A element that is not scroll container must accept but ignore the values of this property. This property must be applied to all input 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.

Name: overscroll-behavior-x, overscroll-behavior-y
Value: contain | none | auto
Initial: auto
Applies to: scroll container elements
Inherited: no
Percentages: N/A
Computed value: as specified
Canonical order: per grammar
Media: visual
Animatable: no

The overscroll-behavior-x property specifies the behavior of the overscroll-behavior in the horizontal direction and the overscroll-behavior-y property specifies the handling of the overscroll-behavior in the vertical direction. When scrolling is performed along both the horizontal and vertical axes at the same time, the overscroll-behavior of each respective axis should be considered independently.

Name: overscroll-behavior
Value: [ contain | none | auto ]{1,2}
Initial: auto auto
Applies to: scroll container elements
Inherited: no
Percentages: n/a
Computed value: see individual properties
Canonical order: per grammar
Media: visual
Animatable: no

The two values specify the behavior in the horizontal and vertical direction, respectively. If only one value is specified, the second value defaults to the same value.

Values have the following meanings:

contain
This value indicates that the element must not perform non-local boundary default actions such as scroll chaining or navigation. The user agent must not perform scroll chaining to any ancestors along the scroll chain regardless of whether the scroll originated at this element or one of its descendants. This value must not modify the behavior of how local boundary default actions should behave, such as overscroll behavior.
none
This value implies the same behavior as contain and in addition this element must also not perform local boundary default actions such as showing any overscroll affordances.
auto
This value indicates that the user agent should perform the usual boundary default action with respect to scroll chaining, overscroll and navigation gestures.

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.

6. Security and Privacy Considerations

There are no known security or privacy 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.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Requirements for Responsible Implementation of CSS

The following sections define several conformance requirements for implementing CSS responsibly, in a way that promotes interoperability in the present and future.

Partial Implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported property values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Implementations of CR-level Features

Once a specification reaches the Candidate Recommendation stage, implementers should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec, and should avoid exposing a prefixed variant of that feature.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-OVERFLOW-3]
David Baron; Elika Etemad; Florian Rivoal. CSS Overflow Module Level 3. 31 July 2018. WD. URL: https://www.w3.org/TR/css-overflow-3/
[CSS-VALUES-4]
Tab Atkins Jr.; Elika Etemad. CSS Values and Units Module Level 4. 10 October 2018. WD. URL: https://www.w3.org/TR/css-values-4/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

Property Index

Name Value Initial Applies to Inh. %ages Ani­mat­able Canonical order Com­puted value Media
overscroll-behavior [ contain | none | auto ]{1,2} auto auto scroll container elements no n/a no per grammar see individual properties visual
overscroll-behavior-x contain | none | auto auto scroll container elements no N/A no per grammar as specified visual
overscroll-behavior-y contain | none | auto auto scroll container elements no N/A no per grammar as specified visual