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: ED
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: Exploring
ED: https://drafts.csswg.org/css-scroll-snap-1/
Editor: Matt Rakow, Microsoft
Editor: Jacob Rossi, Microsoft
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/
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: ''point'' value of 'scroll-snap-type'
At Risk: 'scroll-snap-stop'
Introduction {#intro}
=====================
This section is not normative.
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 a page partially on-screen when panning.
To this end, we introduce scroll snap positions
which enforce the scroll positions that a scroll container’s scrollport may end at
after a scrolling operation has completed.
Module interactions {#placement}
--------------------------------
This module extends the scrolling user interface features defined in [[!CSS21]] section 11.1.
None of the properties in this module apply to the ''::first-line'' and ''::first-letter'' pseudo-elements.
Values {#values}
----------------
This specification follows the
CSS property
definition conventions from [[!CSS21]]. Value types not defined in
this specification are defined in CSS Level 2 Revision 1 [[!CSS21]].
Other CSS modules may expand the definitions of these value types: for
example [[CSS3VAL]], when combined with this module, expands the
definition of the <> value type as used in this specification.
Motivating Examples {#examples}
===============================
In this example, a series of images arranged in a scroll container
are used to build a photo gallery. In this example the scroll container
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 scroll container’s scrollport.
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: center none;
}
.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;
}
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 X 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).
This example builds a paginated document that aligns each page near to (but not exactly on) the edge of the scroll container.
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.
.page {
/* Defines the top of each page as the
edge that should be used for snapping */
scroll-snap-align: none start;
}
.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;
}
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).
Overview {#overview}
====================
This module introduces control over scroll snap positions,
which are scroll positions that produce particular alignments
of content within a scroll container.
Using the 'scroll-snap-type' property on the relevant scroll container,
the author can request a particular bias
for the scrollport to land on a snap position
after scrolling operations.
Snap positions are specified
as a particular alignment ('scroll-snap-align')
of an element’s scroll snap area
(its border bounding box, as modified by 'scroll-snap-margin')
within the scroll container’s snapport
(its scrollport, as reduced by 'scroll-padding').
This is conceptually equivalent to specifying the alignment of
an alignment subject within an alignment container.
A scroll position that satisfies the specified alignment
is a snap position.
The act of adjusting the scroll position
of a scroll container’s scrollport
such that it is aligned to a snap position
is called snapping,
and a scroll container is said to be
snapped to a snap position
if its scrollport’s scroll position
is that snap position
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 snap positions;
this is left up to the user agent.
Snap positions only affect the nearest ancestor scroll container
on the element’s containing block chain.
Capturing Scroll Snap Areas: Properties on the scroll container {#properties-on-the-scroll-container}
======================================================================================================
Scroll Snapping Rules: the 'scroll-snap-type' property {#scroll-snap-type}
--------------------------------------------------------------------------
Name: scroll-snap-type
Value: none | [ x | y | block | inline | both ] [ mandatory | proximity ]?
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: as specified
Animatable: no
In this example, snapping to headings is enabled in the block axis
(the y axis for horizontal writing, x axis for vertical writing):
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 */
}
Scroll Snap Axis: the ''x'', ''y'', ''block'', ''inline'', and ''both'' values
The axis values
specify what axis(es) are affected by snap positions,
and whether snap positions are evaluated independently per axis,
or together as a 2D point.
Values are defined as follows:
Scroll Snap Strictness: the ''scroll-snap-type/none'', ''proximity'', and ''mandatory'' values
The strictness values
(''scroll-snap-type/none'', ''proximity'', ''mandatory'')
specify how strictly
snap positions are enforced on the scroll container
(by forcing an adjustment to the scroll position).
Values are defined as follows:
If specified on a scroll container,
the scroll container is required to be snapped to a snap position
when there are no active scrolling operations.
If a reachable snap position exists
then the scroll container must snap at the termination of a scroll
(if none exist then no snapping occurs).
proximity
If specified on a scroll container,
the scroll container may snap to a snap position
at the termination of a scroll,
at the discretion of the UA given the parameters of the scroll.
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 captures snap positions
if it is a scroll containeror has a value other than ''scroll-snap-type/none'' for 'scroll-snap-type'.
If a box’s nearest snap-position capturing ancestor
on its containing block chain
is a scroll container with a non-''scroll-snap-type/none'' value for 'scroll-snap-type',
that is the box’s scroll snap container.
Otherwise, the box has no scroll snap container,
and its snap positions do not trigger snapping.
If the content or layout of the document changes
(e.g. content is added, moved, deleted, resized)
such that the content of a snapport changes,
the UA must re-evaluate the resulting scroll position,
and re-snap if required.
If the scroll container was snapped before the content change
and that same snap position 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.
Scroll Snapport: the 'scroll-padding' property {#scroll-padding}
----------------------------------------------------------------
Name: scroll-padding
Value: [ <> | <> ]{1,4}
Initial: 0
Applies to: scroll containers
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
The 'scroll-padding' property defines the
optimal viewing region of the scrollport,
the region used as the target region for placing things in view of the user.
It allows the author to exclude regions of the scrollport
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' effectively reduces the region of the viewport
that is considered “viewable” for scrolling operations:
it has no effect on layout,
on the scroll origin or initial position,
or on whether or not an element is considered actually visible,
but it should
affect whether an element or the caret is considered scrolled into view,
and reduce the amount of scrolling for paging operations
(such as using the PgUp and PgDn keys
or triggering equivalent operations from the scrollbar)
so that within the optimal viewing region of the scrollport
the user sees a continuous stream of content.
For a scroll snap container this region also defines
the scroll snapport--
the area of the scrollport that is used as the alignment container
for the scroll snap areas when calculating snap positions.
Values are interpreted as for 'padding',
and specify inward offsets from each edge of the scrollport.
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.
This property is a shorthand property that sets
all of the scroll-padding-* longhands
in one declaration.
Aligning Scroll Snap Areas: Properties on the elements {#properties-on-the-elements}
=====================================================================================
Scroll Snapping Area: the 'scroll-snap-margin' property {#scroll-snap-margin}
-----------------------------------------------------------------------------
Name: scroll-snap-margin
Value: <>{1,4}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length
The 'scroll-snap-margin' property defines
the scroll snap area
that is used for snapping this box to the snapport.
The <> values give outsets
(interpreted as for 'margin' or 'border-image-outset').
The scroll snap area is the rectangular bounding box of the transformed border box,
plus the specified outsets,
axis-aligned in the scroll container’s coordinate space.
Note: This ensures that the scroll snap area is always rectangular
and axis-aligned to the scroll container’s coordinate space.
This property is a shorthand property that sets
all of the scroll-snap-margin-* longhands
in one declaration.
Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
--------------------------------------------------------------------------------
Name: scroll-snap-align
Value: [ none | start | end | center ]{1,2}
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: two keywords
Animatable: no
Since the purpose of scroll snapping is to align content within the viewport
for optimal viewing,
a scroll container cannot be snapped to a snap area
if no part of it is within the snapport.
For example, a snap area is top-aligned to the snapport
if its top edge is coincident with the snapport’s top edge;
however, this scroll position is nonetheless not a valid snap position
if the entire snap area is outside the snapport
(because the required alignment, though satisfied, would not be relevant to the viewer).
Alignment of an off-screen element
is not considered snapping.
Why limit snapping to only when the element is visible?
As the WebKit implementers point out,
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.)
Snapping Boxes that Overflow the Scrollport
If the snap area is larger than the snapport in a particular axis,
then any scroll position in which the snap area covers the snapport,
and the distance between the geometrically previous and subsequent snap positions in that axis
is larger than size of the snapport in that axis,
is a valid snap position in that axis.
The UA may use the specified alignment as a more precise target
for certain scroll operations (e.g. explicit paging).
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 snap area is larger than the snapport,
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 snap position.
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.
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.
Note: If the author had instead set mandatory snap positions
on the headings 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.
Unreachable Snap Positions
If a snap position is unreachable as specified,
such that aligning to it would require scrolling the scroll container’s viewport
past the edge of its scrollable overflow region,
the usedsnap position for this snap area
is the position resulting from scrolling as much as possible
in each relevant axis
toward the desired snap position.
Scroll Snap Limits: the 'scroll-snap-stop' property {#scroll-snap-stop}
--------------------------
Name: scroll-snap-stop
Value: normal | always
Initial: normal
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: as specified
Animatable: no
Media: interactive
This property specifies whether the scroll container
must stop at each snap position it passes,
or may pass multiple snap positions before coming to rest.
Values are defined as follows:
normal
The scroll container may pass by a snap position defined
by this element during the execution of a scrolling operation.
always
The scroll container must not pass by a snap position
defined by this element during the execution of a scrolling operation.
Snapping Mechanics {#snap-concepts}
===================================
The precise model algorithm to select a snap position 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.
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:
: intended end position
::
Common examples of scrolls with only an intended end position 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
: intended direction and end position
::
Common examples of scrolls with both an intended direction and end position include:
* a “fling” gesture,
interpreted with momentum
* programmatically scrolling via APIs such as {{Window/scrollBy()}}
The intended end point of the scroll prior to intervention from features such
as snap points is its natural end-point.
: intended direction
::
Common examples of scrolls with only an intended direction include:
* pressing an arrow key on the keyboard
* a swiping gesture interpreted as a fixed (rather than inertial) scroll
Additionally, because page layouts usually align things vertically and/or horizontally,
UAs sometimes axis-lock a scroll when its direction
is sufficiently vertical or horizontal.
An axis-locked scroll is bound to only scroll along that axis.
This prevents less-precise input mechanisms from drifting in the non-primary axis.
Choosing Snap Positions {#choosing}
-----------------------------------
A scroll container can have many snap areas
scattered throughout its scrollable overflow region.
A naïve algorithm for selecting a snap position
can produce behavior that is unintuitive for users,
so care is required when designing a selection algorithm.
Here are a few pointers that can aid in the selection process:
* Snap positions should be chosen to minimize the distance between the end-point
(or the natural end-point)
and the final snapped scroll position,
subject to the additional constraints listed in this section.
* If a scroll is axis-locked,
any snap positions in the other axis should be ignored
during the scroll.
(However, snap positions in the other axis can still effect the final scroll position.)
* In order to prevent a far-offscreen element
from having difficult-to-understand effects
on the scroll position,
snap positions should be ignored if their elements are far outside of the “corridor”
that the snapport defines as it moves through the scrollable overflow region,
or a hypothetical “corridor” in the direction of a scroll with only an intended direction,
or the snapport after an scroll with only an intended end position.
* User agents must ensure that a user can “escape” a snap position,
regardless of the scroll method.
For example, if the snap type is ''mandatory''
and the next snap position is more than two screen-widths away,
a naïve “always snap to nearest” selection algorithm might “trap” the user
if their end position was only one screen-width away.
Instead, a smarter algorithm that only returned to the starting snap position
if the end-point was a fairly small distance from it,
and otherwise ignored the starting snap position,
would give better behavior.
(This implies that a scroll with only an intended direction
must always ignore the starting snap positions.)
* If a page is navigated to a fragment that defines a target element
(one that would be matched by '':target'',
or the target of {{Element/scrollIntoView()}}),
and that element defines some snap positions,
the user agent should snap to one of that element’s snap positions.
The user agent may do this even when the scroll container has ''scroll-snap-type: none''.
Appendix A: Longhands {#longhands}
==================================
Physical Longhands for 'scroll-padding' {#padding-longhands-physical}
----------------------------------------------------------------------
Name: scroll-padding-top, scroll-padding-right, scroll-padding-bottom, scroll-padding-left
Value: <> | <>
Initial: 0
Applies to: scroll containers
Inherited: no
Percentages: relative to the scroll container’s scrollport
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These longhands of 'scroll-padding' specify
the top, right, bottom, and left edges
of the snapport, respectively.
Flow-relative Longhands for 'scroll-padding' {#padding-longhands-logical}
--------------------------------------------------------------------------
Name: scroll-padding-inline-start, scroll-padding-block-start, scroll-padding-inline-end, scroll-padding-block-end
Value: <> | <>
Initial: 0
Applies to: scroll containers
Inherited: no
Percentages: relative to the scroll container’s scrollport
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These longhands of 'scroll-padding' specify
the block-start, inline-start, block-end, and inline-end edges
of the snapport, respectively.
Name: scroll-padding-block, scroll-padding-inline
Value: [ <> | <> ]{1,2}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: relative to the scroll container’s scrollport
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These shorthands of 'scroll-padding-block-start' + 'scroll-padding-block-end'
and 'scroll-padding-inline-start' + 'scroll-padding-inline-end'
are longhands of 'scroll-padding',
and specify the block-axis and inline-axis edges of the snapport, respectively.
If two values are specified, the first gives the start value
and the second gives the end value.
Physical Longhands for 'scroll-snap-margin' {#margin-longhands-physical}
-------------------------------------------------------------------------
Name: scroll-snap-margin-top, scroll-snap-margin-right, scroll-snap-margin-bottom, scroll-snap-margin-left
Value: <>
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length
These longhands of 'scroll-snap-margin' specify
the top, right, bottom, and left edges
of the scroll snap area, respectively.
Flow-relative Longhands for 'scroll-snap-margin' {#margin-longhands-logical}
-----------------------------------------------------------------------------
Name: scroll-snap-margin-block-start, scroll-snap-margin-inline-start, scroll-snap-margin-block-end, scroll-snap-margin-inline-end
Value: <>
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length
These longhands of 'scroll-snap-margin' specify the
block-start, inline-start, block-end, and inline-end edges
of the scroll snap area, respectively.
Name: scroll-snap-margin-block, scroll-snap-margin-inline
Value: <>{1,2}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: n/a
Media: interactive
Computed value: as specified, with lengths made absolute
Animatable: as length
These shorthands of 'scroll-snap-margin-block-start' + 'scroll-snap-margin-block-end'
and 'scroll-snap-margin-inline-start' + 'scroll-snap-margin-inline-end'
are longhands of 'scroll-snap-margin',
and specify the block-axis and inline-axis edges
of the scroll snap area, respectively.
If two values are specified, the first gives the start value
and the second gives the end value.
Privacy and Security Considerations {#priv-sec}
===============================================
This specification does not expose any information whatsoever
that is not already exposed to the DOM directly;
it just makes scrolling slightly more functional.
There are no new privacy or security considerations.
Acknowledgements {#acknowledgements}
====================================
Many thanks to
David Baron,
Simon Fraser,
Håkon Wium Lie,
Edward O’Connor,
François Remy,
Majid Valpour,
and most especially Robert O’Callahan
for their proposals and recommendations,
which have been incorporated into this document.
Changes {#changes}
==================
Changes since the 23 June 2016 Working Draft include:
Renamed 'scroll-padding' to 'scroll-padding' and extended its uses.
Removed renaming issues because nobody came up with a better alternative to ''scroll-snap-type: mandatory | proximity'' or ''scroll-snap-stop: normal''.
