Overview.src.html

<h1>CSS Scroll Snap Points Module Level 1</h1>

<pre class='metadata'>
Group: csswg
Shortname: css-snappoints
Level: 1
Status: ED
ED: http://dev.w3.org/csswg/css-snappoints/
Editor: Matt Rakow, Microsoft
Editor: Jacob Rossi, Microsoft
Abstract: This module contains features to control panning and scrolling behavior with "snap points".
!Issue Tracking: <a href="http://wiki.csswg.org/spec/css-snappoints">http://wiki.csswg.org/spec/css-snappoints</a>
Ignored Terms: scroll-snap-points-*
</pre>

<h2 id="intro">Introduction</h2>

    <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 offset
    which leaves a page partially on-screen when panning.

    To this end, we introduce scroll snap points
    which enforce the scroll offsets that a <a>scroll container's</a> visual viewport may end at
    after a scrolling operation has completed.

<h3 id="placement">Module interactions</h3>

    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.

<h3 id="values">Values</h3>

    This specification follows the
    <a href="http://www.w3.org/TR/CSS21/about.html#property-defs">CSS property
    definition conventions</a> 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 <<length>> value type as used in this specification.

<h2 id="examples">Motivating Examples</h2>

    <div class="example">
        In this example, a series of images arranged in a <a>scroll container</a>
        are used to build a photo gallery.
        The <a>scroll container</a> is sized to the same size of the images contained therein.
        Using mandatory snap points,
        scrolling will always complete with a snap point aligned to the edge of the <a>scroll container's</a> visual viewport.
        By aligning a snap point at the edge of each image,
        this creates a photo viewer were users can scroll through the images one at a time.

        <pre>
        img {
            width:500px;
        }
        .photoGallery {
            width: 500px;
            overflow-x: auto;
            overflow-y: hidden;
            white-space: nowrap;
            /* Sets up points to which scrolling
               will snap along x-axis */
            scroll-snap-points-x: repeat(100%);
            /* Requires that scrolling always end at a snap point when
               the operation completes (hard snap) */
            scroll-snap-type: mandatory;
        }
        </pre>

        <pre>
            &lt;div class="photoGallery">
                &lt;img src="img1.jpg">
                &lt;img src="img2.jpg">
                &lt;img src="img3.jpg">
                &lt;img src="img4.jpg">
                &lt;img src="img5.jpg">
            &lt;/div>
        </pre>

        <figure>
            <img src="interval_snap_points.png" alt="">

            <figcaption>
                The layout of the scroll container's contents in the example.
                Snap points are set along the x-axis,
                starting at 0px and repeating at intervals of 100% of the scroll container's width.
            </figcaption>
        </figure>
    </div>

    <div class="example">
        This example also builds a photo gallery as in example 1.  However, 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 (such that a repeating snap interval would not be effective).  Using mandatory element-based snap
        points, scrolling will always complete with an image centered in the <a>scroll container's</a> visual viewport.

        <pre>
            img {
                /* Defines the center of each photo as the
                   coordinate that should be used for snapping */
                scroll-snap-coordinate: 50% 50%;
            }
            .photoGallery {
                width: 500px;
                overflow-x: auto;
                overflow-y: hidden;
                white-space: nowrap;
                /* Specifies that each element's snap coordinate should
                   align with the center of the scroll container */
                scroll-snap-destination: 50% 50%;
                /* Requires that scrolling always end at a snap point
                   when the operation completes (hard snap). */
                scroll-snap-type: mandatory;
            }
        </pre>

        <pre>
            &lt;div class="photoGallery">
                &lt;img src="img1.jpg">
                &lt;img src="img2.jpg">
                &lt;img src="img3.jpg">
                &lt;img src="img4.jpg">
                &lt;img src="img5.jpg">
            &lt;/div>
        </pre>

        <figure>
            <img src="element_snap_points.png" alt="">

            <figcaption>
                The layout of the scroll container's contents in the example.
                The snap-destination is horizontally and vertically centered within the scroll container's visual viewport
                (represented by a red X),
                and each element has its snap coordinate horizontally and vertically centered within the element
                (represented by yellow plus signs).
            </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 points instead of mandatory snap points 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 point, then the scroll will be adjusted to
        align the page as specified.

        <pre>
            .page {
                /* Defines the top center of each page as the
                   coordinate that should be used for snapping */
                scroll-snap-coordinate: 50% 0;
            }
            .docScroller {
                width: 500px;
                overflow-x: hidden;
                overflow-y: auto;
                /* Specifies that each element's snap coordinate should
                   align with the center of the scroll container, offset
                   a short distance from the top edge. */
                scroll-snap-destination: 50% 100px;
                 /* Encourages scrolling to end at a snap point when the
                    operation completes, if it is near a snap point */
                scroll-snap-type: proximity;
            }
        </pre>

        <pre>
            &lt;div class="docScroller">
                &lt;div class="page">Page 1&lt;/div>
                &lt;div class="page">Page 2&lt;/div>
                &lt;div class="page">Page 3&lt;/div>
                &lt;div class="page">Page 4&lt;/div>
            &lt;/div>
        </pre>

        <figure>
            <img src="element_snap_points_offset.png" alt="">

            <figcaption>
                The layout of the scroll container's contents in the example.
                The snap-destination is horizontally centered and offset 100px from the top edge with respect to the scroll container's visual viewport
                (represented by a red X),
                and each element has its snap coordinate horizontally centered and top-aligned with respect to the element
                (represented by yellow plus signs).
            </figcaption>
        </figure>
    </div>

<h2 id="definitions">Definitions</h2>

    <dl>
        <dt><dfn export>scroll container</dfn>
        <dd>
            An element which provides a scrolling user interface as described in [[!CSS21]], particularly in the section on overflow.
    </dl>

<h2 id="scroll-snap-type">Scroll Snap Types: the 'scroll-snap-type' property</h2>

    The ''scroll-snap-type'' property is used to define how strictly snap points are enforced on the <a>scroll container</a>, if any are present. It defines how and when snap points are enforced on the visual viewport of the scroll container it is applied to in order to adjust scroll offset.  It intentionally does not specify nor mandate any precise animations or physics used to enforce those snap points; this is left up to the user agent.

    <pre class='propdef'>
    Name: scroll-snap-type
    Value: none | mandatory | proximity
    Initial: none
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: n/a
    Media: interactive
    Computed value: specified value
    Animatable: no
    </pre>

    <dl dfn-type="value" dfn-for="scroll-snap-type">
        <dt><dfn>none</dfn>
        <dd>
            The visual viewport of this <a>scroll container</a> must ignore snap points, if any, when scrolled.

        <dt><dfn>mandatory</dfn>
        <dd>
            The visual viewport of this <a>scroll container</a> is guaranteed to rest on a snap point when there are no active scrolling operations.  That is, it must come to rest on a snap point at the termination of a scroll, if possible.  If the content changes such that the visual viewport would no longer rest on a snap point (e.g. content is added, moved, deleted, resized), the scroll offset must be modified to maintain this guarantee.

        <dt><dfn>proximity</dfn>
        <dd>
            The visual viewport of this <a>scroll container</a> may come to rest on a snap point at the termination of a scroll at the discretion of the UA given the parameters of the scroll.  If the content changes such that the visual viewport would no longer rest on a snap point (e.g. content is added, moved, deleted, resized), the scroll offset may be modified to maintain this guarantee.
    </dl>

    <div class="issue">
        Describe the guarantee as an invariant for better clarity.  Include edge case behavior such as mandatory snap points when there is no satisfiable snap point.
    </div>

<h2 id="scroll-snap-points">Scroll Snap Points: the 'scroll-snap-points-x' and 'scroll-snap-points-y' properties</h2>

    The 'scroll-snap-points-x' and 'scroll-snap-points-y' properties
    are used to define the positioning of snap points
    within the content of the <a>scroll container</a> they are applied to.

    <pre class="propdef">
    Name: scroll-snap-points-x, scroll-snap-points-y
    Value: none | repeat(<<length>>)
    Initial: none
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: relative to same axis of the padding-box of the <a>scroll container</a>
    Media: interactive
    Computed value: specified value, with lengths made absolute
    Animatable: no
    </pre>

    <dl dfn-type="value" dfn-for="snap-points-x, snap-points-y">
        <dt><dfn>none</dfn>
        <dd>
            No snap points are defined by this <a>scroll container</a>.  Contained elements may still define snap points on this <a>scroll container's</a> behalf.

        <dt><dfn>repeat(<<length>>)</dfn>
        <dd>
            Defines an interval at which snap points are defined, starting from the container's relevant <a>start</a> edge.  Only positive lengths are allowed.
    </dl>

    <div class="issue">
        Should there be a way to specify that either end of the scrollable content should have a snap point?
    </div>

<h2 id="scroll-snap-destination">Scroll Snap Destination: the 'scroll-snap-destination' property</h2>

    The 'scroll-snap-destination' property is used to define the x and y coordinate within the <a>scroll container's</a> visual viewport
    which element snap points will align with.

    <pre class="propdef">
    Name: scroll-snap-destination
    Value: <<position>>
    Initial: 0px 0px
    Applies to: <a>scroll containers</a>
    Inherited: no
    Percentages: relative to width and height of the padding-box of the <a>scroll container</a>
    Media: interactive
    Computed value: specified value, with lengths made absolute
    Animatable: yes
    </pre>

    <dl dfn-type="value" dfn-for="scroll-snap-destination">
        <dt><dfn><<position>></dfn>
        <dd>
            Specifies the offset of the snap destination from the start edge of the <a>scroll container's</a> visual viewport.  The first value gives the x coordinate of the snap destination, the second value its y coordinate.
    </dl>

<h2 id="scroll-snap-coordinate">Scroll Snap Coordinate: the 'scroll-snap-coordinate' property</h2>

    The 'scroll-snap-coordinate' property is used to define the x and y coordinate within the element
    which will align with the nearest ancestor <a>scroll container's</a> snap-destination for the respective axis.  In the case that the element has been transformed, the snap coordinate is also transformed in the same way (such that the snap-point is aligned with the element as-drawn).

    <p class="issue">
        How does this work with fragmentation?
    </p>

    <p class="issue">
        Consider alternative naming besides "coordinate".  Consider naming conventions like in Grid Layout for grouping properties on the container vs. items.
    </p>

    <pre class="propdef">
    Name: scroll-snap-coordinate
    Value: none | <<position>>#
    Initial: none
    Applies to: all elements
    Inherited: no
    Percentages: refer to the element's border box
    Media: interactive
    Computed value: specified value, with lengths made absolute
    Animatable: yes
    </pre>

    <dl dfn-type=value dfn-for="scroll-snap-coordinate">
        <dt><dfn>none</dfn>
        <dd>
            Specifies that this element does not contribute a snap point.

        <dt><dfn><<position>>#</dfn>
        <dd>
            Specifies the offset of the snap coordinates from the start edge of the element's border box.  For each pairing, the first value gives the x coordinate of the snap coordinate, the second value its y coordinate.
    </dl>

    <p class="issue">
        How can an alternative box be specified?
    </p>

<h2 class=no-num id="acknowledgments">Acknowledgments</h2>

    Many thanks to lots of people for their proposals and recommendations, some of which are incorporated into this document.