Title: CSS Scroll Snapping Change Proposal
Shortname: css-scroll-snap
Level: 1
Status: UD
Work Status: exploring
Group: CSSWG
ED: https://drafts.csswg.org/css-scroll-snap/
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
Abstract: A brief description of an alternate model for scroll-snapping.
Ignored Terms: snap position, snap positions, scrollable area, scroll-group-align, containing block chain
At Risk: ''point'' value of 'scroll-snap-type'
spec: css-shapes-1; type: value; for:
text: border-box
text: margin-box
spec: css-writing-modes-3; type: dfn
text: start
text: end
Introduction {#intro}
=====================
We think scroll snapping is a great idea, and fully support exposing this functionality through CSS. However, a major weakness of the current spec is the way it conceives snapping on a coordinate model rather than a box model. This requires a lot of manual calculations in figuring out the correct coordinates from the box model; and also makes sensible scroll-snap settings dependent on the relative sizes of the viewport and the snappable contents, causing problems for users are unexpectedly large and/or small screens (a problem commonly ignored by many authors).
This proposal builds off of roc's model, using an area-snapping model to intelligently handle adaptation to multiple screen sizes. It also adds group alignment as a built-in concept, rather than requiring authors to build one in JavaScript.
Use Cases {#use-cases}
======================
Use Case 1: Snapping to the start or middle of each box
e.g. address book (start) or photo album (middle)
1. Snapping to 0.25rem above the top of each heading
:root { scroll-snap-type: proximity; }
h1, h2, h3, h4, h5, h6 {
scroll-snap-align: start;
scroll-snap-margin: 0.25em;
}
2. Snapping to the center of each photo
:root { scroll-snap-type: mandatory; }
img { scroll-snap-align: center; }
Use Case 2: Snapping to boxes (or points) in 2D
e.g. on a map, where you want to snap points of interest to the
center, or a flow-chart diagram, where you want to snap the edges
of each box into the visible area. In both cases, you don't want
objects wholly outside the visible area to influence snapping.
1. Snapping each flow chart entry to within the viewport when it falls near the edge:
:root {
scroll-snap-type: proximity;
}
li {
scroll-snap-align: start;
}
2. Snapping each city on a map to the center of the viewport,
but only once it gets near the center in both dimensions:
:root {
scroll-snap-type: proximity;
}
.city {
scroll-snap-align: center;
}
Use Case 3: Slideshow, where successive slides are arranged horizontally,
and sometimes "detail" slides are placed below the "main" slide for that point.
<div class="slides">
<div class="slide">...</div>
<div class="slide">...</div>
<div class="slide details">
<div class="slide">...</div>
<div class="slide">...</div>
</div>
<div class="slide">...</div>
</div>
<style>
.slides {
display: flex;
flex-flow: row;
scroll-snap-type: mandatory;
overflow-x: scroll;
width: 100vw;
height: 100vh;
}
.slide {
scroll-snap-align: start;
width: 100vw;
min-height: 100vh;
}
.slide.details {
display: flex;
flex-flow: column;
scroll-snap-type: mandatory;
overflow-y: scroll;
}
</style>
| Spec
| Proposal
| Priority
|
| ''scroll-snap-type: none | mandatory | proximity''
| ''scroll-snap-type: none | [ mandatory | proximity ] || [ x | y | block | inline | both | point]''
| High priority
|
''scroll-snap-destination: <>''
''scroll-snap-padding: [ <> | <> ]{1,4}''
|
| | |
On the children:
| Spec
| Proposal
| Priority
|
''scroll-snap-coordinate: <>#''
| ''scroll-snap-align: [ none | start | end | center ]{1,2}''
| High priority
|
| n/a
| ''scroll-snap-margin: <>{1,4}''
| High priority
| | |
Scroll Snapping Model {#snap-model}
=====================
This module introduces control over scroll snap positions,
which are scroll positions that produce particular alignments
of content within a scrollable viewport.
Using the 'scroll-snap-type' property on the relevant scroll container,
the author can request a particular bias
for the viewport to land on a valid snap position
after scrolling operations.
Snap positions are specified
as a particular alignment ('scroll-snap-align')
of a box’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-snap-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 valid 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 {#snap-container}
===========================
Scroll Snapping Rules: the 'scroll-snap-type' property {#snap-type}
----------------------
Name: scroll-snap-type
Value: none | [ proximity | mandatory ] || [ x | y | block | inline | both | point ]
Initial: none
Applies to: all elements
Inherited: no
Percentages: n/a
Computed value: as specified
Animatable: no
The 'scroll-snap-type' property specifies
whether a scroll container is a scroll snap container,
how strictly it snaps,
and which axes are considered.
Issue: We're considering splitting this into subproperties.
Current proposed names are scroll-snap for the current grammar,
scroll-snap-affinity for the proximity/mandatory distinction,
and scroll-snap-axis for the x/y/etc distinction.
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:
- none
-
If specified on a scroll container,
the scroll container
must not snap:
all scroll positions are equally valid.
If specified on a non-scroll container,
this value has no effect.
- proximity
-
If specified on a scroll container,
the scroll container
may snap to a valid snap position
at the termination of a scroll,
at the discretion of the UA given the parameters of the scroll.
If specified on a non-scroll container,
this value “traps” descendant boxes’ snap positions,
preventing them from affecting any ancestor scroll containers.
- mandatory
-
If specified on a scroll container,
the scroll container
is required to be snapped to a valid snap position
when there are no active scrolling operations.
That is, it must snap to a valid snap position
at the termination of a scroll, if any such exist.
(If none exist, then no snapping occurs.)
If specified on a non-scroll container,
this value “traps” descendant boxes’ snap positions,
preventing them from affecting any ancestor scroll containers.
A box captures snap positions
if it is a scroll container
or 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.
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 viewport
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.
For example, if an author wishes to force snapping to the top of each section heading,
s/he could accomplish this in two ways: snapping the headings
h1, h2, h3, h4, h5, h6 { scroll-snap-align: start; } /* snap headings - but not section content */
or snapping the section elements.
section { scroll-snap-align: start; } /* snap entire section - including content */
If the author chooses mandatory snapping of the headings,
and one section is longer than the viewport,
then the reader will have difficulty accessing the content that overflows the screen,
because mandatory snapping does not allow the scroll position to rest
on the content between the snapped headings.
However, if the author chooses mandatory snapping of the section element
(which contains all the content of the section)
then the UA can allow the reader to scroll freely through the entire section
in the cases where the content is longer than the screen.
- x
-
The scroll container axis-snaps to snap positions
in its horizontal axis only.
- y
-
The scroll container axis-snaps to snap positions
in its vertical axis only.
- block
-
The scroll container axis-snaps to snap positions
in its block axis only.
- inline
-
The scroll container axis-snaps to snap positions
in its inline axis only.
- both
-
The scroll container axis-snaps to snap positions
in both of its axises independently
(potentially snapping to different elements in each axis).
- point
-
The scroll container point-snaps to snap positions
in both axises simultaneously,
treating each element’s snap position as a single 2D position
(rather than potentially snapping to different elements in each axis).
If no axis value is specified, then the axis is automatically computed:
* If the scroll container is only scrollable in one axis
(only one axis has its 'overflow' set to ''overflow/auto'' or ''overflow/scroll'')
it axis-snaps in the scrollable axis only.
* Otherwise, it axis-snaps in its block axis only.
If the content or layout of the scroll container changes
(e.g. content is added, moved, deleted, resized),
the UA must re-evaluate, and potentially re-snap if necessary,
the resulting scroll position
once the positions of the content have restabilized.
This can result in no snapping in some cases
(e.g. if there are no nearby snap positions
for a ''proximity''-snapping scroll container).
However, if there was a previously-snapped snap position
(associated with the same element)
that still exists after such changes,
the UA must remain snapped to it--
even if the changes would have placed a different snap position
closer to the current scroll position.
Otherwise, the scroll container must be re-snapped
as if the user had scrolled to its current scroll position
(as an explicit scroll, if no other scroll operation is active).
Scroll Snapport: the 'scroll-snap-padding' property {#snap-padding}
-----------------------
Name: scroll-snap-padding
Value: [ <> | <> ]{1,4}
Initial: 0
Applies to: scroll containers
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
The 'scroll-snap-padding' property 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, points of interest in a map are centered
within the portion of the viewport that does not include the toolbar overlay.
map {
overflow: scroll;
scroll-snap-type: proximity;
scroll-snap-padding: 3em 0 0 0;
}
toolbar {
position: absolute;
margin: 0.5em;
top: 0; left: 0; right: 0;
height: 2em;
}
city {
scroll-snap-align: center;
}
Name: scroll-snap-margin
Value: <>{1,4}
Initial: 0
Applies to: all elements
Inherited: no
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 viewport.
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-alignment}
--------------------------
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
Animatable: no
The 'scroll-snap-align' property specifies
the box's snap position as an alignment of
its snap area (as the alignment subject)
within its snap container's snapport (as the alignment container).
The two values specify snapping alignment
in the inline axis and block axis, respectively.
If only one value is specified, the second value defaults to the same value.
The following example aligns the start edge of the box's
snap area
to the start edge of the scroll container's
snapport:
section { scroll-snap-align: start; }
The following example aligns the center of each city
to the center of the scroll container's
snapport,
snapping only when the city is centered in both axes:
.map { scroll-snap-type: proximity point; }
.map .city { scroll-snap-align: center; }
The following example aligns the center of each photo
to the center of the scroll container's
snapport,
snapping independently in each axis:
.photos { scroll-snap-type: mandatory both; }
img { scroll-snap-align: center; }
- none
-
This box does not define a snap position in the specified axis.
- start
-
Start alignment of this box's scroll snap area
within the scroll container's snapport
is a valid snap position
in the specified axis.
- end
-
End alignment of this box's scroll snap area
within the scroll container's snapport
is a valid snap position
in the specified axis.
- center
-
Center alignment of this box's scroll snap area
within the scroll container's snapport
is a valid snap position
in the specified axis.
If the element's scroll container is point-snapping,
and this property does not specify a valid snap position in both axises
(that is, it contains ''scroll-snap-align/none''),
the element does not contribute any snap positions at all.
For all of these values,
the block or inline axis
is relative to the element's parent's writing mode.
Issue: Is this the correct writing mode to compute against?
Or should it be the scroll container's writing mode?
Why no <> or <> values?
The values here represent alignments
(in the sense of 'align-self' and 'justify-self'),
so are consistent with that syntax.
We chose to use this simpler syntax without lengths or percentages
because the 'scroll-snap-margin' concept already provides length offsets--
but does so in a smarter way, that degrades better on small screens (see above)
because it provides more information (a box, rather than a point) to the UA.
We could have also added lengths here,
but it would provide multiple ways to do the same thing,
which is additional overhead for implementation, testing, and (most importantly) author learning.
It also introduces more room for cascading errors,
and guides authors in the wrong direction--
away from 'scroll-snap-margin'.
Scoping Valid Snap Positions to Visible Boxes
Since the purpose of scroll snapping is to align content within the viewport
for optimal viewing:
in all cases, the specified alignment creates a valid snap position
only if at least part of the snap area 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 alignment is nonetheless not a valid snap position
if the entire snap area is outside the snapport.
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.)
Snapping Boxes that Overflow the Scrollport
If the snap area is larger than the snapport in a particular axis,
and there are no other snap areas within the snapport
that would provide a snap position aligning the overflowing snap area within the snapport,
then any scroll position in which the snap area covers the snapport
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. inertial scrolling or explicit paging).
For example, take the third code fragment in the previous example,
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 you fling out or pull it sufficiently to trigger snapping to a different
snap position.
Unreachable Snap Areas
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 area,
the used snap 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
This property specifies whether the snap position
absorbs all remaining inertia during an inertial scroll,
or allows the inertial scroll to pass multiple snap positions before coming to rest.
Values are defined as follows:
- normal
-
A snap position defined by this element
does not interfere with the inertia
of an inertial scroll that is passing across it,
unless it is the landing snap position.
- always
-
A snap position defined by this element,
when encountered by an inertial scroll,
absorbs all remaining inertia from an inertial scroll,
forcing a stop at this snap position,
exactly as if the scroll had enough inertia to reach the snap position,
but not enough to escape it.
Note: This means that if all snap positions in a scroller
have ''scroll-snap-stop: always'',
an inertial scroll can only move one snap position
per inertial scroll action.
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:
: explicit scrolling
:: A scroll is explicit if it has an intended end position,
but no intended direction.
This includes methods such as:
* 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
: inertial scrolling
:: A scroll is inertial if it has both an intended end position
and an intended direction.
This includes methods such as:
* a "fling" gesture,
released with momentum
(the "intended" end position might be implicitly determined by the UA's scrolling physics,
but the strength of the user's fling still expresses a weak intention
about where the scroll should end up)
* a mousewheel scroll
* programmatically scrolling via APIs such as {{Window/scrollBy()}}
The scroll position that an inertial scroll would naturally land on
without further intervention is the natural end-point.
: directional scrolling
:: A scroll is directional if it has an intended direction,
but no intended end point.
This includes methods such as:
* pressing an arrow key on the keyboard
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,
for example,
a nearly horizontal fling gesture from gradually drifting up or down as well,
because it is very difficult to fling in a precisely horizontal line.
Axis vs Point-Snapping {#snap-dimensions}
-----------------------------------------
Issue: This feature is planned to be removed in the next publication
in order to reduce the feature-set of Level 1.
It is included here for future reference in defining Level 2.
There are two distinct snapping behaviors that a scroll container might engage in:
: axis-snapping
:: If a scroll container is axis-snapping,
its descendants indicate a desired scroll position
in each axis of the scroll container independently,
with no dependent preference for what the other axis's scroll position should be.
Note: This is the “default” type of snap behavior
that most scroll containers will want to use,
and so the ''scroll-snap-type'' property intentionally defaults to it.
Note: An element in an axis-snapping scroll container
can declare two snap positions,
one in each axis.
If one of the element's snap positions is chosen in one axis,
this has no bearing on the other dimension's snap position--
it might be chosen,
or a different element's snap position might be chosen for that axis,
or that axis might not snap at all.
: point-snapping
:: If a scroll container is point-snapping,
its descendants indicate a desired scroll position
in both axises of the scroll container simultaneously--
in other words,
some point in the descendant must be aligned to a corresponding point in the scroll container.
This type of snapping behavior is intended for "two-dimensional" panning-type layouts,
such as cities on a map
(using ''proximity'' 2D snap positions to snap a city to the center of the display when it gets close),
or a tiled image gallery
(using ''mandatory'' 2D snap positions to force each image to be centered on the screen).
In both of these cases,
it would look weird if the horizontal scrolling was aligned to one element
while the vertical was aligned to a different element
(which is the behavior you'd get if the scroll container was axis-snapping).
Choosing Snap Positions {#choosing}
-----------------------------------
A scroll container can have many snap areas
scattered throughout its scrollable area.
A naive 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.
* Point-snapping is all-or-nothing;
if the snap position of an element is chosen to align to,
the scroll container must set its scroll position
according to the element's snap positions in both axises;
the scroll container must not “partially align” to the element
by taking its snap position in one axis
and aligning the other axis according to something else.
* If a scroll is axis-locked and the scroll container is axis-snapping,
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.)
If a scroll is axis-locked and the scroll container is point-snapping,
snap positions should be penalized in the selection process
according to the amount of other-axis scrolling they would cause.
* Snap positions should be ignored if their elements are far outside of the “corridor”
that the snapport defines as it moves through the scrollable area
during an inertial scroll,
or a hypothetical “corridor” in the direction of a directional scroll,
or the snapport after an explicit scroll.
(This is to prevent a far-offscreen element
from having difficult-to-understand effects
on the scroll 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 would “trap” the user
if they were panning with a touch gesture;
a sufficiently large distance would even trap fling scrolling!
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 directional scroll 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''),
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-snap-padding' {#padding-longhands-physical}
--------------------------------------------
Name: scroll-snap-padding-top, scroll-snap-padding-right, scroll-snap-padding-bottom, scroll-snap-padding-left
Value: <> | <>
Initial: 0
Applies to: scroll containers
Inherited: no
Percentage: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These longhands of 'scroll-snap-padding' specify
the top, right, bottom, and left edges
of the snapport,
respectively.
Flow-relative Longhands for 'scroll-snap-padding' {#padding-longhands-logical}
-------------------------------------------------
Name: scroll-snap-padding-inline-start, scroll-snap-padding-block-start, scroll-snap-padding-inline-end, scroll-padding-block-end
Value: <> | <>
Initial: 0
Applies to: scroll containers
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These longhands of 'scroll-snap-padding' specify
the block-start, inline-start, block-end, and inline-end edges
of the snapport, respectively.
Name: scroll-snap-padding-block, scroll-snap-padding-inline
Value: [ <> | <> ]{1,2}
Initial: 0
Applies to: all elements
Inherited: no
Percentages: relative to the corresponding dimension of the scroll container’s scrollport
Computed value: as specified, with lengths made absolute
Animatable: as length, percentage, or calc
These shorthands of 'scroll-snap-padding-block-start' + 'scroll-snap-padding-block-end'
and 'scroll-snap-padding-inline-start' + 'scroll-snap-padding-inline-end'
are longhands of 'scroll-snap-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' {#area-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
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' {#area-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
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
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,
Theresa O'Connor,
François Remy,
Majid Valpour,
potentially some anonymous Microsoft engineers (?),
and most especially Robert O'Callahan
for their proposals and recommendations,
which have been incorporated into this document.