Changes in DOM elements above the visible region of a scrolling box can result in the page moving while the user is in the middle of consuming the content.

This spec proposes a mechanism to mitigate this jarring user experience by keeping track of the position of an anchor node and adjusting the scroll offset accordingly.

This spec also proposes an API for web developers to opt-out of this behavior.

Standardizing this proposal is tracked by a WICG interventions issue.

Introduction

Today, users of the web are often distracted by content moving around due to changes that occur outside the viewport. Examples include script inserting an iframe containing an ad, or non-sized images loading on a slow network.

Historically the browser's default behavior has been to preserve the absolute scroll position when such changes occur. This means that to avoid shifting content, the webpage can attempt to reserve space on the page for anything that will load later. In practice, few websites do this consistently.

Scroll anchoring aims to minimize surprising content shifts. It does this by adjusting the scroll position to compensate for the changes outside the viewport.

The explainer document gives an informal overview of scroll anchoring.

Description

Scroll anchoring works by selecting a DOM node (the anchor node) whose movement is used to determine adjustments to the scroll position.

Anchor Node Selection

Each scrolling box aims to select an anchor node that is deep in the DOM and close to the block start edge of its optimal viewing region.

If the user agent does not support the scroll-padding property, the optimal viewing region of the scrolling box is equivalent to its content area.

The anchor node is either a non-anonymous block box or a text node. The anchor node is always a descendant of the scrolling box. In some cases, a scrolling box may not select any anchor node.

The anchor node selection algorithm for a scrolling box S is as follows:

  1. If S is associated with an element whose computed value of the overflow-anchor property is none, then do not select an anchor node for S.
  2. Otherwise, for each DOM child N of the element or document associated with S, perform the candidate examination algorithm for N in S, and terminate if it selects an anchor node.
The candidate examination algorithm for a candidate DOM node N in a scrolling box S is as follows:
  1. If N is an excluded subtree, or if N is fully clipped in S, then do nothing (N and its descendants are skipped).
  2. If N is fully visible in S, select N as the anchor node.
  3. If N is partially visible:
    1. For each DOM child C of N, perform the candidate examination algorithm for C in S, and terminate if it selects an anchor node.
    2. For each absolute-positioned element A whose containing block is N, but whose DOM parent is not N, perform the candidate examination algorithm for A in S, and terminate if it selects an anchor node.
    3. Select N as the anchor node. (If this step is reached, no suitable anchor node was found among N's descendants.)
    Deeper nodes are preferred to minimize the possibility of content changing inside the anchor node but outside the viewport, which would cause visible content to shift without triggering any scroll anchoring adjustment.

Conceptually, a new anchor node is computed for every scrolling box whenever the scroll position of any scrolling box changes. (As a performance optimization, the implementation may wait until the anchor node is needed before computing it.)

A DOM node N is an excluded subtree if it is an element and any of the following conditions holds:

A DOM node N is fully visible in a scrolling box S if its scroll anchoring bounding rect is entirely within the optimal viewing region of S.

A DOM node N is fully clipped in a scrolling box S if its scroll anchoring bounding rect is entirely outside the optimal viewing region of S.

A DOM node N is partially visible in a scrolling box S if it is neither fully visible in S nor fully clipped in S.

The scroll anchoring bounding rect of a DOM node N is its scrollable overflow rectangle if N is a block box, or the bounding rect of its line boxes if N is a text node.

Scroll Adjustment

If an anchor node was selected, then when the anchor node moves, the browser computes the previous offset y0, and the current offset y1, of the block start edge of the anchor node's scroll anchoring bounding rect, relative to the block start edge of the scrolling content in the block flow direction of the scroller.

It then queues an adjustment to the scroll position of y1 - y0, in the block flow direction, to be performed at the end of the suppression window.

The scroll adjustment is a type of scrolling as defined by CSSOM View Module, and generates scroll events in the manner described there.

Suppression Window

Every movement of an anchor node occurs within a window of time called the suppression window, defined as follows:

The suppression window boundaries should be incorporated into the HTML standard once the scroll anchoring API is stabilized.
More than one anchor node movement may occur within the same suppression window.

At the end of a suppression window, the user agent performs all scroll adjustments that were queued during the window and not suppressed by any suppression trigger during the window.

Suppression Triggers

A suppression trigger is an operation that suppresses the scroll anchoring adjustment for an anchor node movement, if it occurs within the suppression window for that movement. These triggers are:

Suppression triggers exist for compatibility with existing web content that has negative interactions with scroll anchoring due to shifting content in scroll event handlers.

Exclusion API

Scroll anchoring aims to be the default mode of behavior when launched, so that users benefit from it even on legacy content. A CSS property overflow-anchor can disable scroll anchoring in part or all of a webpage (opt out), or exclude portions of the DOM from the anchor node selection algorithm. This property supports the following values when applied to an element E:

The overflow-anchor property was also proposed (with different values) for CSS Sticky Scrollbars, which has now been superseded.

The overflow-anchor property is not inherited.

References

Normative References

[CSSOM-VIEW-1]
Simon Pieters. CSSOM View Module. 17 March 2016. WD. URL: https://drafts.csswg.org/cssom-view/