CSS Overscroll Behavior Module Level 1

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:

• 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.

• When scrolling is performed near the edge of the scroll boundary, the default action may cause both scrolling to the edge of the scroll container and a boundary default action. Calling preventDefault() will not only cancel the boundary default action but also the scroll to the edge of the scrollport.

• 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.

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & 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 CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

2. Motivating Examples

#sidebar {
  overscroll-behavior: contain;
}

html {
  /* only disable pull-to-refresh but allow swipe navigations */
  overscroll-behavior-y: contain;
}

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

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.

4. Overscroll Behavior Properties

The overscroll behavior 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.

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:

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 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.

Note: Programmatic scrolling is clamped and can not trigger any boundary default actions.

4.1. Overscroll and Positioned Elements

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 does 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 must not 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 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 above the scroller isn’t affected by the scroller doing anything at all, including overscroll.

4.2. Physical Longhands for overscroll-behavior

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.

4.3. Flow-relative Longhands for overscroll-behavior

These properties correspond to the overscroll-behavior-x and overscroll-behavior-y properties. The mapping depends on the element’s writing-mode.

5. Security and Privacy Considerations