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.
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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:
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 preventDefault was called causing increased
scroll latency.
The default action for the event may also provide additional behavior that the author does
not want to cancel such as an overscroll affordance. preventDefault doesn’t allow the
content author to cancel only some of the default actions such as scroll chaining.
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. The viewport participates in scroll chaining as the
document’s scrollingElement, both regarding placement in the scroll chain as well as adhering
to the chaining rules applied to it.
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.
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.
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 showing any overscroll affordances.
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.
The overscroll-behavior-x property specifies the overscroll behavior in the horizontal
axis and the overscroll-behavior-y property specifies the overscroll behavior in the
vertical axis. 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.
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:
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
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.