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Tombstone the spec and point everything to the CSSWG draft.
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# Scroll Anchoring

* [Explainer](explainer.md)
* [Draft spec](https://wicg.github.io/ScrollAnchoring)
* [Now a CSSWG specification!](https://drafts.csswg.org/css-scroll-anchoring)

## Implementation status

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# Scroll Anchoring

**Note, Scroll Anchoring is now a CSSWG specification: <https://drafts.csswg.org/css-scroll-anchoring>**

Scroll anchoring is an
[intervention](https://github.com/WICG/interventions)
that adjusts the scroll position to reduce visible content "jumps".
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349 changes: 5 additions & 344 deletions index.html
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<!DOCTYPE html>
<html>
<head>
<title>Scroll Anchoring</title>
<meta charset='utf-8'>
<script src='https://www.w3.org/Tools/respec/respec-w3c-common'
async class='remove'></script>
<script class='remove'>
var respecConfig = {
specStatus: "CG-DRAFT",
shortName: "scrollAnchoring",
edDraftURI: "https://wicg.github.io/ScrollAnchoring",
editors: [
{ name: "Steve Kobes",
url: "",
company: "Google",
companyURL: "https://www.google.com/" },
{ name: "Tab Atkins",
url: "",
company: "Google",
companyURL: "https://www.google.com/" },
],
otherLinks: [{
key: 'Participation',
data: [
{
value: 'GitHub repository',
href: 'https://github.com/WICG/ScrollAnchoring'
}, {
value: 'File a bug / view open issues',
href: 'https://github.com/WICG/ScrollAnchoring/issues'
}, {
value: 'Feedback / discussion in the WICG',
href: 'https://discourse.wicg.io/t/proposal-scroll-anchoring/1752'
}
]
}],
};
</script>
</head>
<body>
<section id='abstract'>
<p>
Changes in DOM elements above the visible region of a
<a href="https://drafts.csswg.org/cssom-view/#scrolling-box">scrolling box</a>
can result in the page moving while the user is in the middle of consuming the content.
</p>
<p>
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.
</p>
<p>
This spec also proposes an API for web developers to opt-out of this behavior.
</p>
</section>
<title>Scroll Anchoring</title>
<meta charset=utf-8>
<h1>Scroll Anchoring Has Migrated to the CSSWG</h1>

<section id='sotd'>
<p>
Standardizing this proposal is tracked by <a href="https://github.com/wicg/interventions/issues/2">a WICG interventions issue</a>.
</p>
</section>

<section id="intro" class="informative">
<h2>Introduction</h2>
<p>
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.
</p>
<p>
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.
</p>
<p>
Scroll anchoring aims to minimize surprising content shifts. It does this by
adjusting the scroll position to compensate for the changes outside the
viewport.
</p>
<p>
The <a href="https://github.com/WICG/ScrollAnchoring/blob/master/explainer.md">explainer
document</a> gives an informal overview of scroll anchoring.
</p>
</section>
<section dfn-for="ScrollAnchoring">
<h2>Description</h2>
<p>
Scroll anchoring works by selecting a DOM node (the <dfn>anchor node</dfn>) whose
movement is used to determine adjustments to the scroll position.
</p>
<section>
<h3>Anchor Node Selection</h3>
<p>
Each <a href="https://drafts.csswg.org/cssom-view/#scrolling-box">scrolling box</a>
aims to select an anchor node that is deep in the DOM and close to the
block start edge of its
<a href="https://drafts.csswg.org/css-scroll-snap/#optimal-viewing-region">optimal viewing region</a>.
</p>
<div class="note">
If the user agent does not support the
<a href="https://drafts.csswg.org/css-scroll-snap/#propdef-scroll-padding">scroll-padding</a>
property, the optimal viewing region of the scrolling box is equivalent to its
<a href="https://www.w3.org/TR/css3-box/#content">content area</a>.
</div>
<p>
The anchor node is either a non-anonymous
<a href="https://www.w3.org/TR/css3-box/#block">block box</a>
or a <a href="https://www.w3.org/TR/dom/#text">text node</a>.
The anchor node is always a <a href="https://www.w3.org/TR/dom/#concept-tree-descendant">descendant</a>
of the <a href="https://drafts.csswg.org/cssom-view/#scrolling-box">scrolling box</a>.
In some cases, a scrolling box may not select any anchor node.
</p>
<p>
The <dfn id="anchoring-algorithm">anchor node selection algorithm</dfn> for a scrolling box S
is as follows:
<ol>
<li>
If S is associated with an element whose computed value of the
<a href="#overflow-anchor">overflow-anchor</a> property is
<a href="#overflow-anchor-none">none</a>, then do not select an anchor node for S.
</li>
<li>
Otherwise, for each DOM child N of the element or document associated with S, perform
the <a href="#candidate-examination">candidate examination algorithm</a> for N in S,
and terminate if it selects an anchor node.
</li>
</ol>
The <dfn id="candidate-examination">candidate examination algorithm</dfn> for a candidate
DOM node N in a scrolling box S is as follows:
<ol>
<li>
If N is an <a>excluded subtree</a>, or if N is <a id="ref-for-fully-clipped-1">fully
clipped</a> in S, then do nothing (N and its descendants are skipped).
</li>
<li>
If N is <a id="ref-for-fully-visible-1">fully visible</a> in S, select N as the anchor node.
</li>
<li>
If N is <a id="ref-for-partially-visible-1">partially visible</a>:
<ol type="a">
<li>
For each DOM child C of N, perform the <a href="#candidate-examination">candidate
examination algorithm</a> for C in S, and terminate if it selects an anchor node.
</li>
<li>
For each <a href="https://drafts.csswg.org/css-position-3/#valdef-position-absolute">absolute-positioned</a> element A whose <a href="https://drafts.csswg.org/css-display-3/#containing-block">containing block</a> is N, but whose
DOM parent is not N, perform the <a href="#candidate-examination">candidate
examination algorithm</a> for A in S, and terminate if it selects an anchor node.
</li>
<li>
Select N as the anchor node. (If this step is reached, no suitable anchor node was
found among N's descendants.)
</li>
</ol>
<div class="note">
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.
</div>
</li>
</ol>
</p>
<p>
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.)
</p>
<p>
A DOM node N is an <dfn id="excluded-subtree">excluded subtree</dfn> if it is an element and any of the following conditions
holds:
<ul>
<li>
N's computed value of the <a href="https://www.w3.org/TR/css-display-3/#propdef-display">display</a> property is <a href="https://www.w3.org/TR/css-display-3/#valdef-display-none">none</a>.
</li>
<li>
N's computed value of the <a href="https://drafts.csswg.org/css-position-3/#propdef-position">position</a> property is <a href="https://drafts.csswg.org/css-position-3/#valdef-position-fixed">fixed</a>.
</li>
<li>
N's computed value of the <a href="https://drafts.csswg.org/css-position-3/#propdef-position">position</a> property is <a href="https://drafts.csswg.org/css-position-3/#valdef-position-absolute">absolute</a> and N's
<a href="https://drafts.csswg.org/css-display-3/#containing-block">containing block</a>
is an ancestor of the scrolling box.
</li>
<li>
N's computed value of the <a href="#overflow-anchor">overflow-anchor</a>
property is <a href="#overflow-anchor-none">none</a>.
</li>
</ul>
</p>
<p>
A DOM node N is <dfn id="fully-visible">fully visible</dfn> in a scrolling box S if its
<a>scroll anchoring bounding rect</a> is entirely within the
<a href="https://drafts.csswg.org/css-scroll-snap/#optimal-viewing-region">optimal viewing region</a>
of S.
</p>
<p>
A DOM node N is <dfn id="fully-clipped">fully clipped</dfn> in a scrolling box S if its
<a>scroll anchoring bounding rect</a> is entirely outside the
<a href="https://drafts.csswg.org/css-scroll-snap/#optimal-viewing-region">optimal viewing region</a>
of S.
</p>
<p>
A DOM node N is <dfn id="partially-visible">partially visible</dfn> in a scrolling box S if it
is neither fully visible in S nor fully clipped in S.
</p>
<p>
The <dfn id="scroll-anchoring-bounding-rect">scroll anchoring bounding rect</dfn> of a DOM node N
is its <a href="https://www.w3.org/TR/css-overflow-3/#scrollable-overflow-rectangle">scrollable overflow rectangle</a>
if N is a <a href="https://www.w3.org/TR/css3-box/#block">block box</a>, or the bounding rect of its
<a href="https://www.w3.org/TR/css-text-3/#line-breaking">line boxes</a> if N is a text node.
</p>
</section>
<section>
<h3>Scroll Adjustment</h3>
<p>
If an anchor node was selected, then when the anchor node moves,
the browser computes the previous offset <code>y0</code>, and the current offset <code>y1</code>,
of the block start edge of the anchor node's <a>scroll anchoring bounding rect</a>,
relative to the block start edge of the scrolling content in the
<a href="https://www.w3.org/TR/css-writing-modes-3/#block-flow-direction">block
flow direction</a> of the scroller.
</p>
<p>
It then queues an adjustment to the scroll position
of <code>y1 - y0</code>, in the block flow direction, to be performed at the end of the
<a>suppression window</a>.
</p>
<p>
The scroll adjustment is a type of
<a href="https://www.w3.org/TR/cssom-view-1/#scrolling-events">scrolling</a> as defined by
CSSOM View Module, and generates
<a href="https://www.w3.org/TR/cssom-view-1/#eventdef-document-scroll">scroll events</a>
in the manner described there.
</p>
<h4>Suppression Window</h4>
<p>
Every movement of an anchor node occurs within a window of time called the
<dfn id="suppression-window">suppression window</dfn>, defined as follows:
<ul>
<li>The suppression window begins at the start of the current iteration of the
<a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#processing-model-8">HTML Processing Model</a> event loop, or at the end of the most recently completed suppression window,
whichever is more recent.</li>
<li>The suppression window ends at the end of the current iteration of the
<a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/webappapis.html#processing-model-8">HTML Processing Model</a> event loop,
or immediately before the next operation whose result or side effects
would differ as a result of a change in the scroll position (for example,
an invocation of
<a href="https://www.w3.org/TR/cssom-view-1/#dom-element-getboundingclientrect">getBoundingClientRect</a>), whichever comes sooner.
</li>
</ul>
<div class="note">
The suppression window boundaries should be incorporated into the HTML standard once the
scroll anchoring API is stabilized.
</div>
More than one anchor node movement may occur within the same suppression window.
</p>
<p>
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
<a>suppression trigger</a> during the window.
</p>
<h4>Suppression Triggers</h4>
<p>
A <dfn id="suppression-trigger">suppression trigger</dfn> 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:
<ul>
<li>
Any change to the computed value of any of the following properties, on any
element in the path from the anchor node to the scrollable element (or
document), inclusive of both:
<ul>
<li><a href="https://drafts.csswg.org/css21/visuren.html#position-props">box offset properties</a></li>
<li><a href="https://drafts.csswg.org/css21/box.html#margin-properties">margin properties</a></li>
<li><a href="https://drafts.csswg.org/css21/box.html#padding-properties">padding properties</a></li>
<li><a href="https://drafts.csswg.org/css21/visudet.html#propdef-width">width</a>,
<a href="https://drafts.csswg.org/css21/visudet.html#propdef-height">height</a>,
<a href="https://drafts.csswg.org/css21/visudet.html#min-max-widths">min-width and max-width</a>,
<a href="https://drafts.csswg.org/css21/visudet.html#min-max-heights">min-height and max-height</a></li>
<li><a href="https://drafts.csswg.org/css-position-3/#propdef-position">position</a></li>
<li><a href="https://drafts.csswg.org/css-transforms-1/#propdef-transform">transform</a></li>
</ul>
</li>
<li>
Any change to the computed value of the
<a href="https://drafts.csswg.org/css-position-3/#propdef-position">position</a>
property on any element within the scrollable element (or document), such that
the element acquires or ceases to have an out-of-flow position
(<a href="https://drafts.csswg.org/css-position-3/#fixed-pos">fixed</a> or
<a href="https://drafts.csswg.org/css-position-3/#abs-pos">absolute</a>).
Note that this trigger applies regardless of whether the modified element is
on the path from the anchor node to the scrollable element.
</li>
</ul>
</p>
<div class="note">
Suppression triggers exist for compatibility with existing web content that has negative
interactions with scroll anchoring due to shifting content in scroll event handlers.
</div>
</section>
</section>
<section>
<h2>Exclusion API</h2>
<p>
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
<dfn id="overflow-anchor"><code>overflow-anchor</code></dfn> 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:
</p>
<ul>
<li>
<dfn><code>overflow-anchor: auto</code></dfn> (the default value) declares
that the DOM subtree rooted at E is eligible to participate in the
<a href="#anchoring-algorithm" id="ref-for-anchoring-algorithm-1">anchor
node selection algorithm</a> for any scrolling box created by E or an ancestor of E.
</li>
<li>
<dfn id="overflow-anchor-none"><code>overflow-anchor: none</code></dfn>
declares that the DOM subtree rooted at E is <em>not</em> eligible to
participate in the <a href="#anchoring-algorithm"
id="ref-for-anchoring-algorithm-1">anchor node selection algorithm</a>
for any scrolling box created by E or an ancestor of E.
</li>
</ul>
<div class="note">
The <code>overflow-anchor</code> property was also proposed (with different values) for
<a href="http://tabatkins.github.io/specs/css-sticky-scrollbars/">CSS Sticky Scrollbars</a>,
which has now been
<a href="https://tabatkins.github.io/specs/css-sticky-scrollbars/#intro">superseded</a>.
</div>
<p>
The <code>overflow-anchor</code> property is not inherited.
</p>
</section>

<h2 id="references">References</h2>
<h3 id="normative">Normative References</h3>
<dl>
<dt id="biblio-cssom-view-1">[CSSOM-VIEW-1]
<dd>Simon Pieters. <a href="https://drafts.csswg.org/cssom-view/">CSSOM View Module</a>. 17 March 2016. WD. URL: <a href="https://drafts.csswg.org/cssom-view/">https://drafts.csswg.org/cssom-view/</a>
</dl>
</body>
</html>
<p>The Scroll Anchoring spec has graduated from the WICG,
and is now <a href="https://drafts.csswg.org/css-scroll-anchoring>a CSSWG spec</a>.

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