Title: CSS Scroll Anchoring Module Level 1
Shortname: css-scroll-anchoring
Level: 1
Group: csswg
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-scroll-anchoring/
TR: https://www.w3.org/TR/css-scroll-anchoring-1/
Previous Version: https://www.w3.org/TR/2020/WD-css-scroll-anchoring-1-20200211/
Editor: Steve Kobes, Google
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Abstract: Changes in DOM elements above the visible region of a scrolling box can result in the page moving
Abstract: while the user is in the middle of consuming the content.
Abstract:
Abstract: This spec proposes a mechanism to mitigate this jarring user experience
Abstract: by keeping track of the position of an anchor node and adjusting the scroll offset accordingly.
Abstract:
Abstract: This spec also proposes an API for web developers to opt-out of this behavior.
spec:css-box; type:dfn; text:content area
spec:css2;
type:dfn; text:line box
spec:css-sizing-3;
type:property;
text:max-height
text:min-height
text:height
text:max-width
text:min-width
text:width
spec: html; urlPrefix: https://html.spec.whatwg.org/multipage/;
urlPrefix: interaction.html
type: dfn;
text:DOM anchor
text:focused area of the document
text:active match; url: fip-active-match
urlPrefix: form-control-infrastructure.html
type: dfn;
text:mutable; url: concept-fe-mutable
spec: html; urlPrefix: https://w3c.github.io/editing/docs/execCommand/;
urlPrefix: index.html
type: dfn;
text:editable
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.
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.
Description
Scroll anchoring attempts to keep the user's view of the document stable across layout changes.
It works by selecting a DOM node (the anchor node)
whose movement is used to determine adjustments to the scroll position.
However, if the scroll container is currently [=scroll snap|snapped=] to an element,
(see [[!CSS-SCROLL-SNAP-1]])
scroll anchoring is limited to adjustments that would be allowed by [=re-snapping=].
Anchor Node Selection
Each scrolling box aims to select an anchor node
that is deep in the DOM
and either should be prioritized as an important DOM node or is close to the
block start edge of its optimal viewing region.
Note: 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.
An anchor node can be any box except one for a non-atomic inline.
The anchor node is always a descendant of the scrolling box.
In some cases, a scrolling box may not select any anchor node.
An element |C| is a viable candidate
for becoming a scroll anchor for a scrolling box |S| if it meets all of the
following criteria:
* |C| is an element that is not a non-atomic inline.
* |C| is partially visible or fully visible in |S|
* |C| is a descendant of |S|
* |C| is not in an excluded subtree
* None of the ancestors of |C| up to |S| are in an excluded subtree
Some elements are considered to be priority candidates for anchor selection:
1. The [=DOM anchor=] of the [=focused area of the document=], if such an anchor
is text editable ([=editable=], [=editing host=], [=mutable=] <{textarea}>, or
[=mutable=] <{input}> with a type that allows text entry).
2. An element containing the current [=active match=] of the
find-in-page user-agent algorithm. If the match spans multiple elements, then
consider only the first such element.
Note that if the priority candidate is a non-atomic inline
element, then instead consider its nearest ancestor element that is not a
non-atomic inline element as the priority candidate.
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 ''overflow-anchor/none'',
then do not select an anchor node for |S|.
2. Otherwise, for each
priority candidate |PC| in order specified,
check if |PC| is a
viable candidate in |S|. If so, select it as an
anchor node and terminate.
3. 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 absolutely-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.)
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.
A DOM node |N| is an
excluded subtree
if it is an element and any of the following conditions holds:
* |N|’s computed value of the 'display' property is ''display/none''.
* |N|’s computed value of the 'position' property is ''position/fixed''.
* |N|’s computed value of the 'position' property is ''position/absolute''
and |N|’s
containing block is an ancestor of the scrolling box.
* |N|’s computed value of the 'overflow-anchor' property is ''overflow-anchor/none''.
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 [[cssom-view-1#scrolling-events#scrolling]] as defined by [[!CSSOM-VIEW]],
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 begins at the start of the current iteration of the
HTML Processing Model event loop,
or at the end of the most recently completed suppression window,
whichever is more recent.
* The suppression window ends at the end of the current iteration of the
HTML Processing Model 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 {{Element/getBoundingClientRect()}}),
whichever comes sooner.
Note: 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:
* 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:
* 'top', 'left', 'right', or 'bottom'
* 'margin' or its longhands
* 'padding' or its longhands
* 'width', 'height', 'min-width', 'max-width', 'min-height', or 'max-height'
* 'position'
* 'transform'
* Any change to the computed value of the 'position' property
on any element within the scrollable element (or document),
such that the element becomes or stops being absolutely positioned.
Note that this trigger applies regardless of whether the modified element is
on the path from the anchor node to the scrollable element.
* The scroll offset of the scrollable element being zero.
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.
Exclusion API
Scroll anchoring aims to be the default mode of behavior when launched,
so that users benefit from it even on legacy content.
'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.
Name: overflow-anchor
Value: auto | none
Initial: auto
Inherited: no
Computed value: specified keyword
Animation type: discrete
Values are defined as follows:
: auto
::
Declares that the element
is potentially eligible to participate in the anchor node selection algorithm
for any scrolling box created by the element or an ancestor.
: none
::
Declares that the element and its descendants
(that aren't nested inside of another scrolling element)
are not eligible to participate in the anchor node selection algorithm
for any scrolling box created by the element or an ancestor.
Note: It is not possible to turn scroll anchoring "back on"
for descendants of a ''overflow-anchor: none'' element.
However, descendant scroll containers automatically "turn it back on"
(for their own scrolling box)
unless they explicitly have ''overflow-anchor: none'' set on them as well.
Note: The overflow-anchor property was also proposed (with different values)
for CSS Sticky Scrollbars,
which has now been superseded.
Privacy and Security Considerations
This specification,
as it only adjusts how we compute scroll positions,
introduces no new privacy or security considerations.
Changes
Changes Since the Feb 11 2020 Working Draft
* Added definitions of [=viable candidate=] and [=priority candidate=].
* Clarified interaction between scroll anchoring and [=scroll snapping=].