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.
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.
Scroll anchoring works by selecting a DOM node (the anchor node) whose movement is used to determine adjustments to the scroll position.
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.
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:
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.
If an anchor node was selected, then when the anchor node moves,
the browser computes the previous offset
y0, and the current offset
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
flow direction of the scroller.
It then queues an adjustment to the scroll position
y1 - y0, in the block flow direction, to be performed at the end of the
The scroll adjustment is a type of scrolling as defined by CSSOM View Module, and generates scroll events in the manner described there.
Every movement of an anchor node occurs within a window of time called the suppression window, defined as follows:
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.
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:
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:
overflow-anchor: auto(the default value) declares that the DOM subtree rooted at E is eligible to participate in the anchor node selection algorithm for any scrolling box created by E or an ancestor of E.
overflow-anchor: nonedeclares that the DOM subtree rooted at E is not eligible to participate in the anchor node selection algorithm for any scrolling box created by E or an ancestor of E.
overflow-anchorproperty was also proposed (with different values) for CSS Sticky Scrollbars, which has now been superseded.
overflow-anchor property is not inherited.