CSS Scroll Snap Module Level 2

1. Introduction

This section is not normative.

This is currently a draft spec over Scroll Snap 1.

Scroll experiences don’t always start at the beginning. Interactions with carousels, swipe controls, and listviews often start somewhere in the middle, and each require JavaScript to set this position on page load. By enabling CSS to specify this scroll start position, both users, page authors and browsers benefit.

In addition to setting an initial scroll position, developers need insights and events into Scroll Snap. Events like which element is snapped on which axis, when the snap event is changing, when snap completes and conveniences for snapping to children programmatically.

1.1. First Layout

This event should follow the Animation code path. When animation objects are created and fire events, this is when a box has it’s first layout.

2. Motivating Examples

.carousel {
    overflow-inline: auto;
    scroll-start: center;
}

<div class="carousel">
    <img src="img1.jpg">
    <img src="img2.jpg">
    <img src="img3.jpg">
    <img src="img4.jpg">
    <img src="img5.jpg">
</div>

.scrollport {
    overflow-block: auto;
}

main {
    scroll-start-target: auto;
}

<div class="scrollport">
    <nav>
        ...
    </nav>
    <main>
        ...
    </main>
</div>

3. Setting Where Scroll Starts

3.1. The scroll-start property

This property is a shorthand property that sets all of the scroll-start-* longhands in one declaration. The first value defines the scroll starting point in the block axis, the second sets it in the inline axis. If the second value is omitted, it defaults to start. If scroll-start-target is set on any child, scroll-start is not used, in favor of using the element as the offset.

Values are defined as follows:

auto

...

<length-percentage [0,∞]>

...

Negative values are invalid. Values corresponding to a length greater than the width/height of the scrollport are valid, but clamped to the width/height of the scrollport.

start

center

end

Equivalent to 0%, 50%, and 100%, respectively.

3.1.1. Interaction with display: none and initial creation

3.1.2. Slow page loading or document streaming behavior

3.1.3. Interaction with fragment navigation

3.1.4. Interaction with place-content

3.1.5. Interaction with "find in page"

3.1.6. Interaction scroll-snap containers

3.1.7. Nested scrollers with scroll-start

3.1.8. Interaction when display is toggled

3.1.9. Interaction with RTL and LTR

3.2. The scroll-start-target property

This property is a shorthand property that sets all of the scroll-start-target-* longhands in one declaration. The first value defines the scroll starting point in the block axis, the second sets it in the inline axis. If the second value is omitted, it defaults to none.

Values are defined as follows:

none

Element is not a scroll-start-target.

auto

Element is used to calculate the scroll-start position, taking into account scroll-padding or scroll-margin , same as a scroll-snap target.

4. Styling Snapped Items

4.1. The Snapped-element Pseudo-class: :snapped

The :snapped pseudo-class matches any scroll snap targets, regardless of axis. The longform physical and logical pseudo-class selectors allow for more finite snapped children styling as they can target an individual axis.

More specific options are defined as follows:

:snapped-x

Matches the child snapped on the horizontal axis.

:snapped-y

Matches the child snapped on the vertical axis.

:snapped-inline

Matches the child snapped on the inline axis.

:snapped-block

Matches the child snapped on the block axis.

Note: Issue #6985
Need to figure out resolution of the initial frame.

5. Snap Events

5.1. snapchanged and snapchanging

CSS scroll snap points are often used as a mechanism to create scroll interactive "selection" components, where selection is determined with JavaScript intersection observers and a scroll end guestimate. By creating a built-in event, the invisible state will become actionable, at the right time, and always correct.

5.1.1. snapchanged

1. when a scrolling operation is completed if, for either the block or inline axis, the element which the snap container is snapped to is different from the element it most recently snapped to in that axis. For snap containers with proximity strictness, the scroll may result in the snap container no longer being snapped to any element. CSS Scroll Snap 1 § 6.2 Choosing Snap Positions describes the method a UA follows when choosing between elements which are snap areas.
2. if there is a change to a snap container’s style such that it goes from having a non-none value for scroll-snap-type to having a none value or vice versa.
3. if, after a layout change, the element to which a snap container is snapped to changes, regardless of whether there is a change in scroll position after the layout change.

Scrolling operations always lead to scrollend events being fired. If a scrolling operation also results in a snapchanged event being fired, the snapchanged event should be fired before the scrollend event.

Each Document has an associated list of pending snapchanged event targets, initially empty.

Each snap container has one snapchangedTargetBlock and one snapchangedTargetInline in the block and inline axes respectively, each of which can either be null if the container is not snapped in that axis or the Element to which the container is snapped.

When asked to update snapchanged targets for a snap container, snapcontainer, run these steps:

1. Let doc be snapcontainer’s associated Document.

2. Let blockTarget be the snapchangedTargetBlock associated with snapcontainer.

3. Let inlineTarget be the snapchangedTargetInline associated with snapcontainer.

4. Let blockSnapchangingTarget be the snapchangingTargetBlock associated with snapcontainer.

5. Let inlineSnapchangingTarget be the snapchangingTargetInline associated with snapcontainer.

6. Let snap targets changed be a boolean flag that is initially false.

7. If blockTarget is not the same element as blockSnapchangingTarget or

   1. Set the snapchangedTargetBlock associated with snapcontainer to blockSnapchangingTarget.

   2. Set snap targets changed to true.

8. If inlineTarget is not the same element as inlineSnapchangingTarget:

   1. Set the snapchangedTargetInline associated with snapcontainer to inlineSnapchangingTarget.

   2. Set snap targets changed to true.

9. If snap targets changed is true:

   1. If snapcontainer is not already in doc’s pending snapchanged event targets:

      1. Append snapcontainer to doc’s pending snapchanged event targets.

Note: When snapping occurs on a scroller (either due to a layout change or a scrolling operation) the snapchangingTargetBlock and snapchangingTargetInline associated with that scroller are updated and represent the current snap targets of that scroller. This allows the update snapchanged targets algorithm to use these elements to determine whether a snapchanged event should be fired.

When asked to dispatch pending snapchanged events for a Document, doc, run these steps:

1. For each item target in doc’s pending snapchanged event targets:

   1. Fire a SnapEvent, snapevent, named snapchanged at target and let snapevent’s snapTargetBlock and snapTargetInline attributes be the snapchangedTargetBlock and the snapchangedTargetInline, respectively, that are associated with target.

2. Empty doc’s pending snapchanged event targets.

5.1.2. snapchanging

• during a scrolling operation, if the element to which a snap container would snap (in either axis) changes, or

• if a layout change occurs such that a snapchanged event is to be dispatched. In this case, as with the scrolling case, the snapchanging event should be dispatched before the snapchanged event.

A scrolling operation might animate towards a particular position (e.g. scrollbar arrow clicks, arrow key presses, "behavior: smooth" programmatic scrolls) or might directly track a user’s input (e.g. touch scrolling, scrollbar dragging). In either case, the user agent chooses an eventual snap target in each axis to which the scroller will snap after the scrolling operation reaches its intended scroll position.

• In the former case, the intended scroll position is the scroll animation’s target scroll offset.

• In the latter case, the intended scroll position is the current scroll offset as determined by the user’s input.

snapchanging aims to let the web page know, as early as possible, that the scrolling operation will result in a change in the element the snap container is snapped to. The user agent should evaluate whether to trigger snapchanging based on the eventual snap target to which the scroller would snap were the scrolling operation to reach its intended scroll position.

Note: Since snapchanging gives the web page hints about future snapping, the snapping hinted at by a snapchanging event might not materialize since it will be possible for subsequent scrolling input to further alter the snap container’s scroll position and result in a different eventual snap target.

snapchanging events are fired before scroll events.

Each Document has an associated list of pending snapchanging event targets, initially empty.

Each snap container has one snapchangingTargetBlock and one snapchangingTargetInlinein the block and inline axes respectively, each of which can either be null if the container is not snapping in that axis or the Element to which the container is snapping.

When asked to update snapchanging targets for a snap container, snapcontainer, given an Element newBlockTarget and an Element newInlineTarget, run these steps:

1. Let doc be snapcontainer’s associated Document.

2. Let blockTarget be the snapchangingTargetBlock that is associated with snapcontainer.

3. Let inlineTarget be the snapchangingTargetInline that is associated with snapcontainer.

4. If newBlockTarget is not the same element as blockTarget:

   1. Set the snapchangingTargetBlock associated with snapcontainer to newBlockTarget.

   2. If snapcontainer is not already in doc’s pending snapchanging event targets,

      1. Append snapcontainer to doc’s pending snapchanging event targets

5. If newInlineTarget is not the same element as inlineTarget:

   1. Set the snapchangingTargetInline associated with snapcontainer to newInlineTarget.

   2. If snapcontainer is not already in doc’s pending snapchanging event targets,

      1. Append snapcontainer to doc’s pending snapchanging event targets.

When asked to dispatch pending snapchanging events for a Document, doc, run these steps:

1. For each item target in doc’s pending snapchanging event targets:

   1. Fire a SnapEvent, snapevent, named snapchanging at target and let snapevent’s snapTargetBlock and snapTargetInline attributes be the snapchangingTargetBlock and the snapchangingTargetInline, respectively, that are associated with target.

2. Empty doc’s pending snapchanging event targets.

5.1.3. Snap Events due to Layout Changes

1. Let newBlockTarget be the element that snapcontainer has snapped to in the block axis or null if it did not snap to any element.

2. Let newInlineTarget be the element that snapcontainer has snapped to in the inline axis or null if it did not snap to any element.

3. Run the steps to update snapchanging targets with newBlockTarget as newBlockTarget and newInlineTarget as newInlineTarget.

4. Run the steps to update snapchanged targets for snapcontainer.

5.2. SnapEvent interface

dictionary SnapEventInit : EventInit {
  Node? snapTargetBlock;
  Node? snapTargetInline;
};

[Exposed=Window]
interface SnapEvent : Event {
  constructor(DOMString type, optional SnapEventInit eventInitDict = {});
  readonly attribute Node? snapTargetBlock;
  readonly attribute Node? snapTargetInline;
};

snapTargetBlock, of type Node, readonly, nullable

The element that the snap container is snapped to in the block axis at the snap position for the associated snap event.

snapTargetInline, of type Node, readonly, nullable

The element that the snap container is snapped to in the inline axis at the snap position for the associated snap event.

For snapchanged events, the snap position is the position already realized by the snap container after a scroll snap. For snapchanging events it is the snap position that the snap container will eventually snap to when the scrolling operation ends.

A SnapEvent should not bubble and should not be cancellable.

Appendix A: Longhands

The physical and logical longhands (and their shorthands) interact as defined in [CSS-LOGICAL-1].

Physical Longhands for scroll-start

...

Flow-relative Longhands for scroll-start

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Flow-relative Longhands for scroll-start-target

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Physical Longhands for scroll-start-target

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