Overview.bs

<pre class='metadata'>
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: fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
Abstract: A brief description of an alternate model for scroll-snapping.
</pre>

<pre class="link-defaults">
spec: css-shapes-1; type: value; for: <shape-box>
	text: border-box
	text: margin-box
</pre>

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}
======================

<div class="example">
	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

		<pre class="lang-css">
		:root { scroll-snap-type: proximity; }
		h1, h2, h3, h4, h5, h6 {
			scroll-snap-align: start;
			scroll-snap-area: 0.25em;
		}
		</pre>

	2. Snapping to the center of each photo

		<pre class="lang-css">
		:root { scroll-snap-type: mandatory; }
		img { scroll-snap-align: center; }
		</pre>
</div>

<div class="example">
	Use Case 2: Snapping to the start or middle of a group of boxes,
	where the number of boxes depends on how many fit in the viewport
	e.g. scrolling image galleries

	1. Snapping to the top of each "page" of address book entries in a list of entries

		<pre class="lang-css">
		:root {
			scroll-snap-type: proximity;
			scroll-group-align: start;
		}
		article {
			scroll-snap-align: group;
		}
		</pre>
</div>

<div class="example">
	Use Case 3: 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:

		<pre class="lang-css">
		:root {
			scroll-snap-type: proximity;
		}
		li {
			scroll-snap-align: edges;
		}
		</pre>

	2. Snapping each city on a map to the center of the viewport,
		but only once it gets near the center in both dimensions:

		<pre class="lang-css">
		:root {
			scroll-snap-type: proximity;
		}
		.city {
			scroll-snap-align: center;
		}
		</pre>
</div>

<div class="example">
	Use Case 4: Slideshow, where successive slides are arranged horizontally,
	and sometimes "detail" slides are placed below the "main" slide for that point.

	<pre class="lang-html">
		&lt;div class="slides">
			&lt;div class="slide">...&lt;/div>
			&lt;div class="slide">...&lt;/div>
			&lt;div class="slide details">
				&lt;div class="slide">...&lt;/div>
				&lt;div class="slide">...&lt;/div>
			&lt;/div>
			&lt;div class="slide">...&lt;/div>
		&lt;/div>
		&lt;style>
		.slides {
			display: flex;
			flex-flow: row;
			scroll-snap-type: mandatory;
			overflow-x: scroll;
			width: 100vw;
			height: 100vh;
		}
		.slide {
			scroll-snap-align: edges;
			width: 100vw;
			min-height: 100vh;
		}
		.slide.details {
			display: flex;
			flex-flow: column;
			scroll-snap-type: mandatory;
			overflow-y: scroll;
		}
		&lt;/style>
	</pre>
</div>

Overview of Change {#proposal}
==============================

On the scroll container:

<table class=data>
	<thead>
		<tr>
			<th>Spec
			<th>Proposal
			<th>Priority
	<tbody>
		<tr>
			<td>'scroll-snap-type'
			<td>'scroll-snap-type' (no change)
			<td>High priority
		<tr>
			<td>''scroll-snap-destination: <<position>>''
			<td>''scroll-snap-destination: auto | <<position>>''
			<td>Keep this property only if needed for compat.
		<tr>
			<td>'scroll-snap-points-x'
			<td>dropped
			<td>High priority to drop. :)
		<tr>
			<td>'scroll-snap-points-y'
			<td>dropped
			<td>High priority to drop. :)
		<tr>
			<td>n/a
			<td>''scroll-group-align: [ start | end | edges ]{1,2} | <<position>>#''
			<td>Low priority
</table>

On the children:

<table class=data>
	<thead>
		<tr>
			<th>Spec
			<th>Proposal
			<th>Priority
	<tbody>
		<tr>
			<td>''scroll-snap-coordinate: <<position>>#''
			<td>''scroll-snap-align: [ none | start | end | edges ]{1,2} | <<position>> | group''
			<td>High priority; simpler version adds a center keyword to the bracketed set instead of allowing the full <<position>># syntax inherited from 'scroll-snap-coordinate'.
				''group'' keyword is low-priority.
		<tr>
			<td>n/a
			<td>''scroll-snap-area: [ border-box | margin-box ] || <<length>>{1,4}''
			<td>High priority
		<!--
		<tr>
			<td>n/a
			<td>''scroll-snap-scope: infinite | finite''
			<td>Only ''finite'' is really necessary to handle all use cases, however UAs might want ''infinite'' as the initial value for performance reasons.
		-->
</table>

Element-based Snapping {#element}
======================

Scroll Snapping Area: the 'scroll-snap-area' property {#scroll-snap-area}
---------------------

	<pre class="propdef">
	Name: scroll-snap-area
	Value: [ border-box | margin-box ] || <<length>>{1,4}
	Initial: border-box
	Applies to: all elements
	Inherited: no
	Computed value: as specified, with lengths made absolute
	Animatable: yes, if ''border-box''/''margin-box'' are constant
	Media: visual
	</pre>

	Specifies the box that is used for snapping.
	<<length>> values give offsets (similar to 'margin' or 'border-image-outset').

	Note: This functionality effectively replaces 'scroll-snap-destination',
	in a way that allows for more control in mixed-content environments
	(each element can specify its own offsets from the viewport edges)
	and also plays better with smaller viewports (see [[#small-viewports]]).

Scroll Snapping Alignment: the 'scroll-snap-align' property {#scroll-snap-align}
--------------------------

	<pre class="propdef">
	Name: scroll-snap-align
	Value: [ none | start | end | edges | center ]{1,2}
	Initial: none
	Applies to: all elements
	Inherited: no
	Computed value: as specified
	Animatable: no
	Media: visual
	</pre>

	Specifies the element's snap position as an alignment of
	its box within the viewport.

	The first value gives alignment in the inline axis;
	the second value gives alignment in the block axis.
	If one value is specified, it is duplicated.

	Issue: ''center'' keyword can be replaced by <<position>># to maintain compat, if necessary.

### Snap Alignment in Small Viewports ### {#small-viewports}

	The snapped position of a box is given by its scroll-snap-align	property.
	However,if the scroll-snap-area is larger than the viewport...

	*	Inertial scrolling (and scroll-adjustment caused by snapping)
		continues to align to the snap-position as normal.

	*	For explicit/programmatic (non-fling) scrolling:

		*	While the area fully fills the viewport in a given axis,
			snapping is ignored in that axis:
			the container can be scrolled arbitrarily and will not react.

		*	If the container is scrolled such that the area
			no longer fully fills the viewport in an axis,
			the area acts as if it has both-edges snapping in that axis,
			resisting outward scrolling until you fling out
			or pull it sufficiently to trigger snapping to a different snap-position
			(with either proximity or mandatory behavior as appropriate).

	As an example, imagine a photo as the area, or a slide in a slideshow.
	You want 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.

<!--
### Combining 1D and 2D Snap Alignments ### {#combo-snapping}

	The following algorithm defines
	how to combine 1D (''edges'', ''start'', or ''end'')
	and 2D (''center'' or <<position>>)
	snap alignments
	in the same scroller:

	1. Find the closest block-axis snapping edge in the scroll direction.
	2. Find the closest inline-axis snapping edge in the scroll direction.
	3. Compute the "edge displacement" as the euclidean distance between the current scroll position
		and what the scroll position would be if the scroll container were snapped to the aforementioned two lines.
	4. Find the closest snap *point* in the scroll direction,
		measured by euclidean distance from the current scroll position.
		The difference is the "point displacement".
	5. If the edge displacement is smaller than the point displacement, snap to those edges.
		Otherwise, snap to the point.

	Issue: Think about how mandatory/finite would work when there's nothing within range to snap to.
-->


Scope of Snapping Influence {#scope}
===========================

	Issue: Current spec doesn't define how to select which snap-point to snap to.
	See <a href="https://lists.w3.org/Archives/Public/www-style/2015Jul/0325.html">https://lists.w3.org/Archives/Public/www-style/2015Jul/0325.html</a>
	for a proposal to ignore snap positions far outside the viewport.

	Issue: UAs should be encouraged to ignore snap positions that require scrolling in two dimensions
	when a one-dimensional scroll is triggered.

	Issue: Define that snap-point selection is based on the final scroll position that the scroll physics would land the scroller in after a fling.

<!--
Scoping Snaplines to Visible Objects: the 'scroll-snap-scope' property {#scroll-snap-scope}
---------------------------

	<pre class="propdef">
	Name: scroll-snap-scope
	Value: infinite | finite
	Initial: infinite
	Applies to: all elements
	Inherited: no
	Computed value: as specified
	Animatable: no
	Media: visual
	</pre>

	When ''finite'' snapping is enabled,
	the "gravitational field" of a snap	alignment is two-dimensional:
	distance to the snap position is calculated for both dimensions at once.

	In other words, if the snapping radius of influence is <var>r</var>,
	in infinite	snapping the box snaps along the y axis
	whenever it is within <var>r</var> of its snapped y position,
	regardless of its x position.
	But in finite snapping,
	the box snaps along the y axis
	whenever it is within <var>r</var> of its snapped position
	in both dimensions.

	<div class="example">
		For example, a small box is snapped to the center of the viewport.
		It only snaps whenever it is < <var>r</var> distance in any direction
		from its snap position in both dimensions.
		In other words, it snaps whenever sqrt(<var>d<sub>x</sub></var><sup>2</sup> + <var>d<sub>y</sub></var><sup>2</sup>) &le; <var>r</var>
		for <var>d<sub>x</sub></var>,<var>d<sub>y</sub></var> as distance to the snapped position in the x and y dimensions, respectively.
	</div>

	<div class="example">
		As another example, a small box is snapped to the edges of the viewport.
		It only snaps whenever matching edges are within <var>r</var> of the respective viewport edges,
		so e.g. whenever its top edge approaches the top of the viewport,
		or its left edge approaches the left of the viewport;
		but there is no snapping effect if those edges are > <var>r</var> outside the viewport.
	</div>

	Issue: This feature can be safely deferred to a future level, if necessary.
	Alternately it can be dropped and ''finite'' snapping can be the default.
	(We can't think of a use case for the infinite snapping model,
	except perhaps UA performance.)
-->

Group-based Snapping {#group}
========================

Collects all snapping areas of group-snapped boxes,
segments them into groups that will fit within the viewport,
then creates snap areas that capture
the specified alignment of each such group.
(Note that such areas may overlap,
if group-snapped boxes are arranged in an overlapping pattern.)

This is a simple form of "scrolling by pages".

<div class="example">
	Use Case 1: Snapping to the top of each "page" of address book entries in a list of entries.

	<pre class="lang-css">
	:root {
		scroll-snap-type: proximity;
		scroll-group-align: start;
	}
	article {
		scroll-snap-align: group;
	}
	</pre>
</div>

<div class="example">
	Use Case 2: Scrolling an article to the first paragraph that hasn't been completely read.

	<pre class="lang-css">
	article {
		scroll-snap-type: proximity;
		scroll-group-align: start;
	}
	article > * {
		scroll-snap-align: group;
	}
	</pre>
</div>

<div class="example">
	Use Case 3: Scrolling image gallery, a la Pinterest, where images are packed tightly on the page.

	<pre class="lang-css">
	.gallery {
		scroll-snap-type: proximity;
		scroll-group-align: center;
	}
	.gallery > img {
		scroll-snap-align: group;
	}
	</pre>
</div>

Turning On Group Snapping: the ''group'' value of 'scroll-snap-align' {#scroll-snap-align-group}
-------------------------

	<pre class="propdef partial">
	Name: scroll-snap-align
	New values: group
	</pre>

	The <dfn value for=scroll-snap-align>group</dfn> value
	specifies that this element's scroll snap area should be group-aligned to the viewport.

Aligning the Group: the 'scroll-snap-group' property {#scroll-snap-group}
-----------------

	<pre class="propdef">
	Name: scroll-snap-group
	Value: [ start | end | edges | center ]{1,2}
	Initial: start
	Applies to: all elements
	Inherited: no
	Computed value: as specified
	Animatable: no
	Media: visual
	</pre>

	Specifies the alignment of a group-snapped group's area within the viewport.