csswg-drafts/css-snappoints/Overview.src.html at c9a9e2268e63f76b1b0fe63f159d3630cd698251 · xfq/csswg-drafts · GitHub

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<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 scroll container 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</a>.
        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</a>.

        <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;
                /* Denotes that snap points will be defined
                   by the elements contained within. */
                scroll-snap-points-x: elements;
                /* 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 snap-destination is horizontally and vertically centered within the scroll container
                (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;
                /* Denotes that snap points will be defined
                   by the elements contained within. */
                scroll-snap-points-y: elements;
                /* 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
                (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 the physics characteristics of panning and scrolling, if any. It defines how and when snap points are enforced on the scroll container it is applied to.

    <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>
            This <a>scroll container</a> must ignore snap points, if any, when scrolled.

        <dt><dfn>mandatory</dfn>
        <dd>
            This <a>scroll container</a> must come to rest on a snap point at the termination of a scroll, if possible.

        <dt><dfn>proximity</dfn>
        <dd>
            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.
    </dl>

    <p class="issue">
        Could this be defined on a per-snap-point basis?   What does it mean to mix types of snap points within a single scroller?  Alternatively, should this be definable separately for each axis?
    </p>

    <p class="issue">
        Need to consider the "snap-area" behavior described by fantasai.  Maybe define this as repelling snap...lines?  guardrails?  bumpers?  boundaries?  limits?  at the boundary?
    </p>

    <p class="issue">
        If we do some behavior like snap-area, what happens when near an edge -- try to align with the edge or not?
    </p>

    <p class="issue">
        What happens in zooming scenarios, e.g. if the images become larger than the viewport?  Snapping makes less sense in this case.  Is this addressable with a snap-area type of effect?
    </p>

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

    <p class="issue">
        Specify behavior of mandatory in a broader range of scenarios that induce scrolling or change content -- e.g. element is removed or moved.  Does the scroller scroll to the nearest valid snap point?
    </p>

<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,
    or to declare the scroll container as snapping to elements.

    <pre class="propdef">
    Name: scroll-snap-points-x, scroll-snap-points-y
    Value: [ <<length>># ,]? repeat(<<length>>) | <<length>># | elements
    Initial: repeat(100%)
    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>[ <<length>># ,]? repeat(<<length>>)</dfn>
        <dd>
            Defines a list of starting snap points and an interval at which additional snap points are defined, starting from the final snap point of the list.
            The first <<length>># list is the set of explicitly defined snap points, and the repeat(<<length>>) function defines the interval to repeat thereafter.

        <dt><dfn><<length>>+</dfn>
        <dd>
            Defines one or more discrete snap points for the <a>scroll container</a>,
            measured as lengths from the container's relevant <a>start</a> edge.

        <dt><dfn>elements</dfn>
        <dd>
            Declares that the snap points for this scroll container
            will be determined by the elements contained within.
            In particular, by the combination of the element boxes' positions in layout
            and their corresponding scroll-snap-coordinate properties,
            in conjunction with this element's scroll-snap-destination property.
    </dl>

    <p class="issue">
        With the split-axis approach, the list and interval can result in unexpected behavior in 2d scrollers.  For example, if you only want two diagonally-positioned snap points at (x1, y1) and (x2, y2), you will instead get (x1, y1), (x1, y2), (x2, y1), and (x2, y2).
    </p>

    <p class="issue">
        Is the "elements" value needed?  Is it important to avoid mixing list/interval with element snap points?
    </p>

    <p class="issue">
        If we do stick with a value like "elements", is there a better name for it (e.g. if we can snap to non-elements like fragments).
    </p>

    <p class="issue">
        Express as position instead of length unit?
    </p>

    <p class="issue">
        Is the list format required, if we have element snapping?  If yes, need a scenario justification.  If no, remove.
    </p>

<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</a>
    which element snap points will align with
    when the ''elements'' value is used for the 'scroll-snap-points-*' property.

    <pre class="propdef">
    Name: scroll-snap-destination
    Value: <<length>>{2}
    Initial: 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><<length>>{2}</dfn>
        <dd>
            Specifies the offset of the snap destination from the leading edge of the scroll container.  The first value gives the x coordinate of the snap destination, the second value its y coordinate.
    </dl>

    <p class="issue">
        Clarify "leading edge" -- want this to respond appropriately to writing modes
    </p>

<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
    when the ''elements'' value is used for the 'scroll-snap-points-*' property
    of the nearest ancestor <a>scroll container's</a> respective axis.

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

    <p class="issue">
        Consider alternative naming besides "coordinate".
    </p>

    <pre class="propdef">
    Name: scroll-snap-coordinate
    Value: none | <<length>>{2}#
    Initial: none
    Applies to: all elements
    Inherited: no
    Percentages: refer to the element box used by the box-sizing property
    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><<length>>{2}#</dfn>
        <dd>
            Specifies the offset of the snap coordinates from the leading edge of the element box used by the box-sizing property.  For each pairing, the first value gives the x coordinate of the snap coordinate, the second value its y coordinate.
    </dl>

    <p class="issue">
        Don't tie this to box-sizing.
    </p>

    <p class="issue">
    </p>

<h2 id="CSSOM">CSSOM</h2>

    <p class="issue">
        Need to define how to read back points.
    </p>

    <p class="issue">
        How to navigate directly to a snap point (e.g. with a hash or similar) -- e.g. enable direct linking to a specific page in a document
    </p>

    <p class="issue">
        Supplementary APIs (e.g. snap-to-next, snap-to-previous, jump-to-N)
    </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.