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<pre class="metadata">
Title: CSS Scroll Snap Module Level 1
Group: csswg
Shortname: css-scroll-snap
TR: https://www.w3.org/TR/css-scroll-snap-1/
Level: 1
Status: CR
Deadline: 2019-05-01
Previous Version: https://www.w3.org/TR/2019/CR-css-scroll-snap-1-20190131/
Previous Version: https://www.w3.org/TR/2018/CR-css-scroll-snap-1-20180814/
Previous Version: https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20171214/
Previous Version: https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20170824/
Previous Version: https://www.w3.org/TR/2017/CR-css-scroll-snap-1-20170209/
Previous Version: https://www.w3.org/TR/2016/CR-css-scroll-snap-1-20161020/
Previous Version: https://www.w3.org/TR/2016/WD-css-scroll-snap-1-20160623/
Previous Version: https://www.w3.org/TR/2016/WD-css-snappoints-1-20160329/
Previous Version: https://www.w3.org/TR/2015/WD-css-snappoints-1-20150326/
Work Status: Testing
ED: https://drafts.csswg.org/css-scroll-snap-1/
Editor: Matt Rakow, Microsoft, w3cid 62267
Editor: Jacob Rossi, Microsoft, w3cid 45616
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Abstract: This module contains features to control panning and scrolling behavior with “snap positions”.
Ignored Terms: containing block chain, scroll position, scrollport
At Risk: 'scroll-snap-stop'
Status Text:
A test suite and an implementation report will be produced during the
CR period.
</pre>
<pre class=link-defaults>
spec: selectors-4; type: selector; text: :target
spec: css-pseudo-4; type: selector;
text: ::first-line
text: ::first-letter
</pre>
Introduction {#intro}
=====================
<em>This section is not normative.</em>
Popular UX paradigms for scrollable content frequently employ paging through content,
or sectioning into logical divisions.
This is especially true for touch interactions
where it is quicker and easier for users to quickly pan through a flatly-arranged breadth of content
rather than delving into a heirarchical structure through tap navigation.
For example, it is easier for a user to view many photos in a photo album
by panning through a photo slideshow view
rather than tapping on individual photos in an album.
However, given the imprecise nature of scrolling inputs
like touch panning and mousewheel scrolling,
it is difficult for web developers to guarantee a well-controlled scrolling experience,
in particular creating the effect of paging through content.
For instance, it is easy for a user to land at an awkward scroll position
which leaves an item partially on-screen when panning.
To this end, this module introduces <a>scroll snap positions</a>
which enforce the scroll positions that a <a>scroll container’s</a> scrollport may end at
after a scrolling operation has completed.
Also, to offer better control over paging and scroll positioning
even when snapping is off,
this module defines the 'scroll-padding' property
for use on all <a>scroll containers</a>,
to adjust the <a>scroll container</a>’s <a>optimal viewing region</a>
for the purpose of paging and scroll-into-view operations.
Similarly the 'scroll-margin' property can be used on any box
to adjust its visual area
for the purpose of scroll-into-view operations.
Module interactions {#placement}
--------------------------------
This module extends the scrolling user interface features defined in [[!CSS2]] section 11.1.
None of the properties in this module apply to the ''::first-line'' and ''::first-letter'' pseudo-elements.
Value Definitions {#values}
-----------------
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> 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 <a>CSS-wide keywords</a> keywords as their property value.
For readability they have not been repeated explicitly.
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Motivating Examples {#examples}
===============================
<div class="example">
In this example, a series of images arranged in a <a>scroll container</a>
are used to build a photo gallery. In this example the <a>scroll container</a>
is larger than the photos contained within (such that multiple images may be seen simultaneously), and the image
sizes vary. Using mandatory element-based snap
positions, scrolling will always complete with an image centered in the <a>scroll container’s</a> scrollport.
<pre class="lang-css">
img {
/* Specifies that the center of each photo
should align with the center of the scroll
container in the X axis when snapping */
scroll-snap-align: none center;
}
.photoGallery {
width: 500px;
overflow-x: auto;
overflow-y: hidden;
white-space: nowrap;
/* Requires that the scroll position always be
at a snap position when the scrolling
operation completes. */
scroll-snap-type: x mandatory;
}
</pre>
<pre class="lang-html">
<div class="photoGallery">
<img src="img1.jpg">
<img src="img2.jpg">
<img src="img3.jpg">
<img src="img4.jpg">
<img src="img5.jpg">
</div>
</pre>
<figure>
<img src="images/element_snap_positions.png" alt="">
<figcaption>
The layout of the scroll container’s contents in the example.
The snapport is represented by the red rectangle, and the snap area is represented by the yellow rectangle. Since the scroll-snap-align is “center” in the inline (horizontal) axis, a snap position is established at each scroll position which aligns the X-center of the snapport (represented by a red dotted line) with the X-center of a snap area (represented by a yellow dotted line).
</figcaption>
</figure>
</div>
<div class="example">
This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the <a>scroll container</a>.
This allows the previous page to “peek” in from above in order to make the user aware that they are not yet at the top of the document.
Using proximity snap positions instead of mandatory snap positions allows the user to stop halfway through a page (rather than forcing them
to snap one page at a time). However, if a scrolling operation would finish near a snap position, then the scroll will be adjusted to
align the page as specified.
<pre class="lang-css">
.page {
/* Defines the top of each page as the
edge that should be used for snapping */
scroll-snap-align: start none;
}
.docScroller {
width: 500px;
overflow-x: hidden;
overflow-y: auto;
/* Specifies that each element’s snap area should
align with a 100px offset from the top edge. */
scroll-padding: 100px 0 0;
/* Encourages scrolling to end at a snap position when the
operation completes, if it is near a snap position */
scroll-snap-type: y proximity;
}
</pre>
<pre class="lang-html">
<div class="docScroller">
<div class="page">Page 1</div>
<div class="page">Page 2</div>
<div class="page">Page 3</div>
<div class="page">Page 4</div>
</div>
</pre>
<figure>
<img src="images/element_snap_positions_offset.png" alt="">
<figcaption>
The layout of the scroll container’s contents in the example.
The snapport is represented by the red rectangle
(inset from the top by 100px due to the scroll-padding),
and the snap area is represented by the yellow rectangle.
Since the scroll-snap-align is “start” in the Y axis,
a snap position is established at each scroll position
which aligns the Y-start of the snapport
(represented by a red dotted line)
with the Y-start of a snap area
(represented by a yellow dotted line).
</figcaption>
</figure>
</div>
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Scroll Snap Model {#overview}
=============================
This module defines controls for
<dfn lt="scroll snap position" local-lt="snap position">scroll snap positions</dfn>,
which are scroll positions that produce particular alignments
of content within a scroll container.
Using the 'scroll-snap-type' property on the relevant <a>scroll container</a>,
the author can request a particular bias
for the scrollport to land on a <a>snap position</a>
after scrolling operations
(including programmatic scrolls such as the {{Window/scrollTo()}} method).
<a>Snap positions</a> are specified
as a particular alignment ('scroll-snap-align')
of an element’s <a>scroll snap area</a>
(its border bounding box, as modified by 'scroll-margin')
within the <a>scroll container</a>’s <a>snapport</a>
(its scrollport, as reduced by 'scroll-padding').
This is conceptually equivalent to specifying the alignment of
an <a>alignment subject</a> within an <a>alignment container</a>.
A scroll position that satisfies the specified alignment
is a <a>snap position</a>.
The act of adjusting the scroll position
of a scroll container’s scrollport
such that it is aligned to a snap position
is called <dfn lt="snap | snapping | snapped">snapping</dfn>,
and a <a>scroll container</a> is said to be
<a>snapped</a> to a <a>snap position</a>
if its scrollport’s scroll position
is that <a>snap position</a>
and there is no active scrolling operation.
The CSS Scroll Snap Module
intentionally does not specify nor mandate
any precise animations or physics used to enforce <a>snap positions</a>;
this is left up to the user agent.
<a>Snap positions</a> only affect the nearest ancestor <a>scroll container</a>
on the element’s <a>containing block chain</a>.
Capturing Scroll Snap Areas: Properties on the scroll container {#properties-on-the-scroll-container}
======================================================================================================
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Scroll Snapping Rules: the 'scroll-snap-type' property {#scroll-snap-type}
--------------------------------------------------------------------------
<pre class="propdef">
Name: scroll-snap-type
Value: none | [ x | y | block | inline | both ] [ mandatory | proximity ]?
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: specified keyword(s)
Animation type: discrete
</pre>
The 'scroll-snap-type' property specifies
whether a <a>scroll container</a> is a <a>scroll snap container</a>,
how <a href="#snap-strictness">strictly</a> it <a>snaps</a>,
and <a href="#snap-axis">which axes</a> are considered.
If no strictness value is specified, ''proximity'' is assumed.
<div class="example">
In this example, snapping to headings is enabled in the <a>block axis</a>
(the y axis for horizontal writing, x axis for vertical writing):
<pre>
html {
scroll-snap-type: block; /* applied to main document scroller */
}
h1, h2, h3, h4, h5, h6 {
scroll-snap-align: start; /* snap to the start (top) of the viewport */
}
</pre>
</div>
UAs must apply the 'scroll-snap-type' value set on the root element
to the document viewport.
Note that, unlike 'overflow',
'scroll-snap-type' values are <em>not</em> propagated from HTML <{body}>.
<h4 id="snap-axis">
Scroll Snap Axis: the ''x'', ''y'', ''scroll-snap-type/block'', ''scroll-snap-type/inline'', and ''both'' values</h4>
The <dfn noexport lt="axis value">axis values</dfn>
specify what axis(es) are affected by <a>snap positions</a>,
and whether <a>snap positions</a> are evaluated independently per axis,
or together as a 2D point.
Values are defined as follows:
<dl dfn-type=value dfn-for="scroll-snap-type">
<dt><dfn>x</dfn>
<dd>
The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
in its horizontal axis only.
<dt><dfn>y</dfn>
<dd>
The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
in its vertical axis only.
<dt><dfn>block</dfn>
<dd>
The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
in its block axis only.
<dt><dfn>inline</dfn>
<dd>
The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
in its inline axis only.
<dt><dfn>both</dfn>
<dd>
The <a>scroll container</a> <a>snaps</a> to <a>snap positions</a>
in both of its axes independently
(potentially snapping to different elements in each axis).
</dl>
<h4 id="snap-strictness">
Scroll Snap Strictness: the ''scroll-snap-type/none'', ''proximity'', and ''mandatory'' values</h4>
The <dfn noexport lt="strictness value">strictness values</dfn>
(''scroll-snap-type/none'', ''proximity'', ''mandatory'')
specify how strictly
<a>snap positions</a> are enforced on the <a>scroll container</a>
(by forcing an adjustment to the scroll position).
Values are defined as follows:
<dl dfn-type="value" dfn-for="scroll-snap-type">
<dt><dfn>none</dfn>
<dd>
If specified on a <a>scroll container</a>,
the <a>scroll container</a> must not <a>snap</a>.
<dt><dfn>mandatory</dfn>
<dd>
If specified on a <a>scroll container</a>,
the <a>scroll container</a> is required to be <a>snapped</a> to a snap position
when there are no active scrolling operations.
If a valid <a>snap position</a> exists
then the scroll container must <a>snap</a> at the termination of a scroll
(if none exist then no <a>snapping</a> occurs).
<dt><dfn>proximity</dfn>
<dd>
If specified on a <a>scroll container</a>,
the <a>scroll container</a> may <a>snap</a> to a snap position
at the termination of a scroll,
at the discretion of the UA given the parameters of the scroll.
</dl>
Advisement:
Authors should use mandatory snap positions with consideration of
varyingly-sized screens and (if applicable) varying-sized content.
In particular, although access to snapped elements larger than the scrollport
is handled by the UA,
if authors assign mandatory snapping to non-adjacent siblings,
content in between can become inaccessible
in cases where it is longer than the screen.
A box <dfn export>captures snap positions</dfn>
if it is a <a>scroll container</a>
<em>or</em> has a value other than ''scroll-snap-type/none'' for 'scroll-snap-type'.
If a box’s nearest <a lt="captures snap positions">snap-position capturing</a> ancestor
on its <a>containing block chain</a>
is a <a>scroll container</a> with a non-''scroll-snap-type/none'' value for 'scroll-snap-type',
that is the box’s <dfn export local-lt="snap container">scroll snap container</dfn>.
Otherwise, the box has no <a>scroll snap container</a>,
and its <a>snap positions</a> do not trigger <a>snapping</a>.
<h4 id="re-snap">
Re-snapping After Layout Changes</h4>
If the content or layout of the document changes
(e.g. content is added, moved, deleted, resized)
such that the content of a <a>snapport</a> changes,
the UA must re-evaluate the resulting <a>scroll position</a>,
and re-snap if required.
If the <a>scroll container</a> was <a>snapped</a> before the content change
and that same <a>snap position</a> still exists
(e.g. its associated element was not deleted),
the scroll container must be re-snapped to that same snap position
after the content change.
Scrolling required by a re-snap operation to a new or different box
must behave and animate the same way as
any other scroll-into-view operation,
including honoring controls such as 'scroll-behavior'.
Scrolling behavior for re-snapping to the same box as before
however, is UA-defined.
The UA may, for example,
when snapped to the start of a section,
choose not to animation the scroll to the section’s new position
as content is dynamically added earlier in the document
in order to create the illusion of not scrolling.
<div class="example">
In the following example,
the log console,
when initially loaded and as each message is added to the bottom,
remains snapped to the bottom of the content
unless the user has scrolled away from that edge:
<pre class="lang-css">
.log {
scroll-snap-type: proximity;
align-content: end;
}
.log::after {
display: block;
content: "";
scroll-snap-align: end;
}
</pre>
The rules create a single [=scroll snap area=]
represented by the ''::after'' pseudo-element,
positioned at the very bottom of a [=scroll snap container=].
If the user scrolls “near” the bottom,
the container will snap to it.
If more content is dynamically added to the container,
it’ll remain snapped to it
(because scroll containers are required
to re-snap to the same scroll snap area
if it still exists after any changes).
However, if the user has scrolled to somewhere else in the logs,
it won’t do anything at all.
</div>
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Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
----------------------------------------------------------------
<pre class="propdef shorthand">
Name: scroll-padding
Value: [ auto | <<length-percentage>> ]{1,4}
Initial: auto
Applies to: <a>scroll containers</a>
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: per side, either the keyword ''scroll-padding/auto'' or a computed <<length-percentage>> value
Animation type: by computed value type
</pre>
This property specifies
(for all <a>scroll containers</a>, not just <a>scroll snap containers</a>)
offsets that define the
<dfn export>optimal viewing region</dfn> of a scrollport:
the region used as the target region for placing things in view of the user.
This allows the author to exclude regions of the <a>scrollport</a>
that are obscured by other content
(such as fixed-positioned toolbars or sidebars)
or simply to put more breathing room
between a targetted element and the edges of the scrollport.
The 'scroll-padding' property is a <a>shorthand property</a> that sets
all of the <a href="#longhands"><css>scroll-padding-*</css> longhands</a>
in one declaration,
assigning values to the longhands representing each side
exactly as the 'padding' property does for its longhands.
Values have the following meanings:
<dl dfn-for="scroll-padding, scroll-padding-inline, scroll-padding-inline-start, scroll-padding-inline-end, scroll-padding-block, scroll-padding-block-start, scroll-padding-block-end" dfn-type=value>
: <dfn><<length-percentage>></dfn>
::
Defines an inward offset from the corresponding edge of the [=scrollport=].
When the scrollport is the root scroller, the offset is relative to the layout viewport.
<div class="note">
Though the [=scrollport=] is defined as the visual viewport of the scroll container,
scroll padding offsets are relative to the layout viewport when the scroll container is
the root scroller.
</div>
: <dfn>auto</dfn>
::
Indicates that the offset for the corresponding edge of the [=scrollport=] is UA-determined.
This should generally default to a used length of ''0px'',
but UAs may use heuristics to detect when a non-zero value is more appropriate.
<div class=example>
For example, a UA could detect when a ''position:fixed'' element
is being used as an opaque unscrollable “header”
that obscures the content below it,
and resolve the top offset to the height of that element
so that a “page down” operation (such as pressing <kbd>PgDn</kbd>)
automatically scrolls by one “visible page” of content.
</div>
</dl>
These offsets reduce the region of the <a>scrollport</a>
that is considered “viewable” <em>for scrolling operations</em>:
they have no effect on layout,
on the scroll origin or initial position,
or on whether or not an element is considered actually <em>visible</em>,
but should
affect whether an element or the caret is considered scrolled into view
(e.g. for targetting or focusing operations),
and reduce the amount of scrolling for paging operations
(such as using the <kbd>PgUp</kbd> and <kbd>PgDn</kbd> keys
or triggering equivalent operations from the scrollbar)
so that within the <a>optimal viewing region</a> of the <a>scrollport</a>
the user sees a continuous stream of content.
For a <a>scroll snap container</a> this region also defines
the <dfn local-lt="snapport">scroll snapport</dfn>--
the area of the scrollport that is used as the <a>alignment container</a>
for the <a>scroll snap areas</a> when calculating <a>snap positions</a>.
<div class="example">
In this example, 'scroll-padding' is used to center slideshow images
within the portion of the scrollport
that is not obscured by a fixed-position toolbar.
<pre class="lang-css">
html {
overflow-x: auto;
overflow-y: hidden;
scroll-snap-type: x mandatory;
scroll-padding: 0 500px 0 0;
}
.toolbar {
position: fixed;
height: 100%;
width: 500px;
right: 0;
}
img {
scroll-snap-align: none center;
}
</pre>
</div>
UAs must apply the 'scroll-padding' values set on the root element
to the document viewport.
(Note that, unlike 'overflow',
'scroll-padding' values are <em>not</em> propagated from HTML <{body}>.)
Aligning Scroll Snap Areas: Properties on the elements {#properties-on-the-elements}
=====================================================================================
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Scroll Snapping Area: the 'scroll-margin' property {#scroll-margin}
-------------------------------------------------------------------
<pre class="propdef">
Name: scroll-margin
Value: <<length>>{1,4}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: per side, an absolute length
Animation type: by computed value type
</pre>
This property is a <a>shorthand property</a> that sets
all of the <a href="#longhands"><css>scroll-margin-*</css> longhands</a>
in one declaration,
assigning values to the longhands representing each side
exactly as the 'margin' property does for its longhands.
Values represent outsets defining the
<dfn lt="scroll snap area" local-lt="snap area">scroll snap area</dfn>
that is used for snapping this box to the snapport.
The <a>scroll snap area</a> is determined by taking the transformed border box,
finding its rectangular bounding box
(axis-aligned in the <a>scroll container’s</a> coordinate space),
then adding the specified outsets.
Note: This ensures that the <a>scroll snap area</a> is always rectangular
and axis-aligned to the <a>scroll container’s</a> coordinate space.
If a page is navigated to a fragment that defines a target element
(one that would be matched by '':target'',
or the target of {{scrollIntoView()}}),
the UA should use the element's <a>scroll snap area</a>,
rather than just its border box,
to determine which area of the <a>scrollable overflow area</a>
to bring into view,
<em>even when snapping is off
or not applied on this element</em>.
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Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
--------------------------------------------------------------------------------
<pre class="propdef">
Name: scroll-snap-align
Value: [ none | start | end | center ]{1,2}
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: two keywords
Animation type: discrete
</pre>
The 'scroll-snap-align' property specifies
the box’s <a>snap position</a> as an alignment of
its <a>snap area</a> (as the <a>alignment subject</a>)
within its <a>snap container’s</a> <a>snapport</a> (as the <a>alignment container</a>).
The two values specify the snapping alignment
in the <a>block axis</a> and <a>inline axis</a>, respectively,
as determined by the [=snap container=]’s [=writing mode=].
If only one value is specified, the second value defaults to the same value.
Values are defined as follows:
<dl dfn-type="value" dfn-for="scroll-snap-align">
<dt><dfn>none</dfn>
<dd>
This box does not define a <a>snap position</a> in the specified axis.
<dt><dfn>start</dfn>
<dd>
Start alignment of this box’s <a>scroll snap area</a>
within the <a>scroll container</a>’s <a>snapport</a>
is a <a>snap position</a>
in the specified axis.
<dt><dfn>end</dfn>
<dd>
End alignment of this box’s <a>scroll snap area</a>
within the <a>scroll container</a>’s <a>snapport</a>
is a <a>snap position</a>
in the specified axis.
<dt><dfn>center</dfn>
<dd>
Center alignment of this box’s <a>scroll snap area</a>
within the <a>scroll container</a>’s <a>snapport</a>
is a <a>snap position</a>
in the specified axis.
</dl>
Start and end alignments are resolved
with respect to the [=writing mode=] of the [=snap container=]
unless the [=scroll snap area=] is larger than the [=snapport=],
in which case they are resolved with respect to the [=writing mode=] of the box itself.
(This allows items in a container to have consistent snap alignment in general,
while ensuring that ''scroll-snap-align/start'' always aligns the item
to allow reading its contents from the beginning.)
<h4 id="snap-scope">
Scoping Valid Snap Positions to Visible Boxes</h4>
Since the purpose of scroll snapping is to align content within the <a>scrollport</a>
for optimal viewing,
a scroll position cannot be considered a valid <a>snap position</a>
if <a>snapping</a> to it would leave the contributing <a>snap area</a>
entirely outside the <a>snapport</a>,
even if it otherwise satisfies the required alignment of the <a>snap area</a>.
<div class="example">
For example, a <a>snap area</a> is top-aligned to the <a>snapport</a>
if its top edge is coincident with the <a>snapport</a>’s top edge;
and this would be considered a valid <a>snap position</a>
for block-axis start-aligned snapping of that <a>snap area</a>
<em>if at least part of the <a>snap area</a> is on-screen</em>.
If the entire <a>snap area</a> is outside the <a>snapport</a>, however,
then the <a>scroll container</a> cannot be considered to be <a>snapped</a>
because the required alignment, though satisfied, would not be relevant to the viewer.
<figure>
<pre class="ascii-art">
╔════viewport════╗┈┈┈┈┈┈┈┈┌──────────────┐
║ ┌─────┐ ┌──┐ ║ │ top-snapping │
║ ├──┐ │ └──┘ ║ │ element │
║ └──┴──┘ ║ │ │
╚════════════════╝ │ │
└──────────────┘
</pre>
<figcaption>
Alignment of an off-screen element
is not considered <a>snapping</a>.
</figcaption>
</figure>
</div>
<details class="note">
<summary>Why limit snapping to only when the element is visible?</summary>
As the <a href="https://www.webkit.org/blog/4017/scroll-snapping-with-css-snap-points/">WebKit implementers point out</a>,
extending a snap edge infinitely across the canvas
only allows for snapping gridded layouts,
and produces odd behavior for the user
when off-screen elements do not align
with on-screen elements.
(If this requirement is onerous for implementers however,
we can default to a gridded behavior
and introduce a switch to get smarter behavior later.)
</details>
Note: Although ''scroll-snap-type: both'' evaluates [=snap positions=] independently in each axis,
<a href="#choosing">choosing</a> of a [=snap position=] in one axis
can be influenced by [=snap positions=] in the other axis.
For example, snapping in one axis
may push off-screen the [=snap area=] that the other axis would otherwise align to,
making its [=snap position=] invalid and therefore unchooseable.
<h4 id="snap-overflow">
Snapping Boxes that Overflow the Scrollport</h4>
If the <a>snap area</a> is larger than the <a>snapport</a> in a particular axis,
then any scroll position in which the <a>snap area</a> covers the <a>snapport</a>,
and the distance between the geometrically previous and subsequent <a>snap positions</a> in that axis
is larger than size of the <a>snapport</a> in that axis,
is a valid <a>snap position</a> in that axis.
The UA may use the specified alignment as a more precise target
for certain scroll operations (e.g. explicit paging).
<div class="example">
For example, take the first example in [[#examples]],
which had a photo as the area.
The author wants mandatory snapping from item to item,
but if the item happens to be larger than your viewport,
you want to be able to scroll around the whole thing once you’re over it.
Since the <a>snap area</a> is larger than the <a>snapport</a>,
while the area fully fills the viewport,
the container can be scrolled arbitrarily
and will not try to snap back to its aligned position.
However, if the container is scrolled such that the area
no longer fully fills the viewport in an axis,
the area resists outward scrolling
until it is scrolled sufficiently
to trigger snapping to a different <a>snap position</a>.
</div>
<div class=example>
For another example,
mandatory top-snapping on nested <{section}> elements
can produce large snapping areas
(from large top-level sections)
potentially filled with smaller snapping areas
(from the subsections).
When the subsections are small enough,
they snap normally;
when they're longer,
the viewer can scroll arbitrarily within them,
or within a large segment of the top-level section that has no subsections to snap to.
<figure>
<pre class=ascii-art>
┌─ top-level section ─┐ ━┓
│ │ 1┃
│ │ ┃
│ │ ━┩
│ │ ┆
│ │ ┆
│┌─── sub-section ───┐│ ╯ ━┓
│└───────────────────┘│ 2┃
│┌─── sub-section ───┐│ ━┓ ┃
││ ││ 3┃ ━┛
│└───────────────────┘│ ┃
│┌─── sub-section ───┐│ ━┛ ━┓
│└───────────────────┘│ 4┃
│┌─── sub-section ───┐│ ━┓ ┃
││ ││ 5┃ ━┛
││ ││ ┃
││ ││ ━┩
││ ││ ┆
││ ││ ┆
││ ││ ┆
│└───────────────────┘│ ┆
└─────────────────────┘ ╯
</pre>
<figcaption>
In the figure above,
the five numbered viewports
represent the five snap positions
associated with the top-level section
and its four subsections.
Because the first and last snap positions are part of ranges taller than the viewport,
the viewer is allowed to scroll freely
between the top and bottom of each range.
</figcaption>
</figure>
Note: If the author had instead set mandatory snap positions
on the <em>headings</em> of each section
(rather than the sections themselves),
the contents of the first and fifth sections
would be partially inaccessible to the user,
as the heading snap area does not extend to cover the whole section.
This is why it's a bad idea to use mandatory snap positions
on elements that might be widely spaced apart.
</div>
<h4 id="unreachable">
Unreachable Snap Positions</h4>
If a <a>snap position</a> is unreachable as specified,
such that aligning to it would require scrolling the <a>scroll container</a>’s viewport
past the edge of its <a>scrollable overflow area</a>,
the <em>used</em> <a>snap position</a> for this <a>snap area</a>
is the position resulting from scrolling <em>as much as possible</em>
in each relevant axis
toward the desired <a>snap position</a>.
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Scroll Snap Limits: the 'scroll-snap-stop' property {#scroll-snap-stop}
--------------------------
<pre class="propdef">
Name: scroll-snap-stop
Value: normal | always
Initial: normal
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: specified keyword
Animation type: discrete
</pre>
When scrolling with an intended direction,
the <a>scroll container</a> can “pass over” several possible <a>snap positions</a>
(that would be valid to snap to,
if the scrolling operation used the same direction
but a lesser distance)
before reaching the natural endpoint of the scroll operation
and selecting its final <a>scroll position</a>.
The 'scroll-snap-stop' property allows such a possible <a>snap position</a>
to “trap” the scrolling operation,
forcing the <a>scroll container</a> to stop
before the scrolling operation would naturally end.
Values are defined as follows:
<dl dfn-type=value dfn-for=scroll-snap-stop>
<dt><dfn>normal</dfn>
<dd>
The <a>scroll container</a> may pass over a <a>snap position</a> defined
by this element during the execution of a scrolling operation.
<dt><dfn>always</dfn>
<dd>
The <a>scroll container</a> must not pass over a <a>snap position</a>
defined by this element during the execution of a scrolling operation;
it must instead snap to the first of this element's <a>snap positions</a>.
</dl>
This property has no effect on scrolling operations with only an <a>intended end position</a>,
as they do not conceptually “pass over” any <a>snap positions</a>.
Snapping Mechanics {#snap-concepts}
===================================
The precise model algorithm to select a <a>snap position</a> to snap to
is intentionally left mostly undefined,
so that user agents can take into account sophisticated models of user intention and interaction
and adjust how they respond over time,
to best serve the user.
This section defines some useful concepts to aid in discussing scroll-snapping mechanics,
and provides some guidelines for what an effective scroll-snapping strategy might look like.
User agents are encouraged to adapt this guidance
and apply their own best judgement
when defining their own snapping behavior.
It also provides a small number of behavior requirements,
to ensure a minimum reasonable behavior that authors can depend on
when designing their interfaces with scroll-snapping in mind.
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Types of Scrolling Methods {#scroll-types}
------------------------------------------
When a page is scrolled,
the action is performed with
an intended end position
and/or an intended direction.
Each combination of these two things
defines a distinct category of scrolling,
which can be treated slightly differently:
: <dfn export>intended end position</dfn>
::
<div class="example">
Common examples of scrolls with only an <a>intended end position</a> include:
* a panning gesture,
released without momentum
* manipulating the scrollbar “thumb” explicitly
* programmatically scrolling via APIs such as {{Window/scrollTo()}}
* tabbing through the document’s focusable elements
* navigating to an anchor within the page
* homing operations such as the <kbd>Home</kbd>/<kbd>End</kbd> keys
</div>
: <dfn export>intended direction and end position</dfn>
::
<div class="example">
Common examples of scrolls with both an <a>intended direction and end position</a> include:
* a “fling” gesture,
interpreted with momentum
* programmatically scrolling via APIs such as {{Window/scrollBy()}}
* paging operations such as the <kbd>PgUp</kbd>/<kbd>PgDn</kbd> keys (or equivalent operations on the scrollbar)
</div>
The intended end point of the scroll prior to intervention from features such
as snap points is its <dfn noexport>natural end-point</dfn>.
: <dfn export>intended direction</dfn>
::
<div class="example">
Common examples of scrolls with only an <a>intended direction</a> include:
* pressing an arrow key on the keyboard (or equivalent operations on the scrollbar)
* a swiping gesture interpreted as a fixed (rather than inertial) scroll
</div>
Additionally, because page layouts usually align things vertically and/or horizontally,
UAs sometimes <dfn export>axis-lock</dfn> a scroll when its direction
is sufficiently vertical or horizontal.
An <a>axis-locked</a> scroll is bound to only scroll along that axis.
This prevents less-precise input mechanisms from drifting in the non-primary axis.
Note: This specification only applies to scrolling methods supported by the user agent;
it does not require the user agent to support any particular input or scrolling method.
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