Title: CSS Overflow Module Level 5
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-5/
Shortname: css-overflow
Group: csswg
Level: 5
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Robert Flack, Google, flackr@google.com, w3cid 98451
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
	It builds on the CSS Overflow Module Level 4,
	adding an appendix containing an experimental exploration
	of redirecting overflow by fragmentation.

type: dfn; spec:css-multicol-1; text:overflow column
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-writing-modes-4; type:dfn; text:start
spec:css-writing-modes-4; type:dfn; text:end

url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;
url: https://www.w3.org/TR/CSS21/visudet.html#strut; type: dfn; text: strut;

Introduction

Overflow Controls: the 'scroll-marker-group' property and pseudo-elements

	Name: scroll-marker-group
	Value: none | before | after
	Initial: none
	Applies to: [=scroll containers=]
	Inherited: no
	Computed value: specified value
	Animation Type: discrete
	Canonical Order: per grammar
	

none

The [=scroll container=] does not create a '::scroll-marker-group' pseudo-element.

before

The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element whose box is an immediate preceding sibling to its [=originating element=].

after

The [=scroll container=] generates a ''::scroll-marker-group'' pseudo-element whose box is an immediate following sibling to its [=originating element=].

The ''::scroll-marker-group'' pseudo-element

		/* The generation of ::scroll-marker pseudo-elements cannot invalidate layout outside of this pseudo-element. */
		::scroll-marker-group { contain: size !important; }
	

The ''::scroll-marker'' pseudo-element

Appendix A: Redirection of Overflow

In CSS Level 1 [[CSS1]], placing more content than would fit inside an element with a specified size was generally an authoring error. Doing so caused the content to extend outside the bounds of the element, which would likely cause that content to overlap with other elements.

CSS Level 2 [[CSS2]] introduced the 'overflow' property, which allows authors to have overflow be handled by scrolling, which means it is no longer an authoring error. It also allows authors to specify that overflow is handled by clipping, which makes sense when the author's intent is that the content not be shown. This was further refined in the CSS Overflow Module Level 3 [[CSS-OVERFLOW-3]].

However, scrolling is not the only way to present large amounts of content, and may even not be the optimal way. After all, the codex replaced the scroll as the common format for large written works because of its advantages.

This specification introduces a mechanism for Web pages to specify that an element of a page should handle overflow through pagination rather than through scrolling.

This specification also extends the concept of overflow in another direction. Instead of requiring that authors specify a single area into which the content of an element must flow, this specification allows authors to specify multiple fragments, each with their own dimensions and styles, so that the content of the element can flow from one to the next, using as many as needed to place the content without overflowing.

In both of these cases, implementations must break the content in the block-progression dimension. Implementations must do this is described in the CSS Fragmentation Module [[!CSS-BREAK-3]].

Channeling Overflow: the 'continue' property

		Name: continue
		New Values: overflow | paginate | fragments
		Initial: auto
		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
		Inherited: no
		Percentages: N/A
		Computed value: see below
		Animation type: discrete
	

auto

''continue/auto'' may only occur as a computed value if the element is a CSS Region other than the last one in a region chain. Content that doesn't fit is pushed to the next region of the chain. In all other cases, ''continue/auto'' computes to one of the other values. Issue: this is different from the definition in [[css-overflow-4#continue]], where the specified value is the computed value. Which is model better?

overflow

Content that doesn't fit overflows, according to the 'overflow' property

paginate

Content that doesn't fit paginates. This creates a paginated view inside the element similar to the way that 'overflow: scroll' creates a scrollable view. See paginated overflow Note: Print is effectively "continue: paginate" on the root.

fragments

content that doesn't fit causes the element to copy itself and continue laying out. See fragment overflow.

1. On elements or pseudo elements with layout containment (see [[!CSS-CONTAIN-1]]), if the specified value is ''continue/auto'' or ''continue/fragments'' then the computed value is ''continue/overflow''.
2. Otherwise, if the specified value is ''continue/auto''
   1. On a CSS Region other than the last one in a region chain, the computed value is ''continue/auto''
   2. On a page the computed value is ''continue/paginate''
   3. On a fragment box the computed value is ''continue/fragments''
   4. Otherwise, the computed value is ''continue/overflow''
3. Otherwise, if the specified value is ''continue/fragments''
   1. On a page the computed value is ''continue/paginate''
   2. Otherwise, the computed value is the specified value
4. In all other cases, the computed value is the specified value

Paginated overflow

@media (overflow-block: paged), (overflow-block: optional-paged) {
	:root {
		continue: paginate;
	}
}

The current implementation of paginated overflow uses the overflow/overflow-x/overflow-y properties rather than the overflow-style property as proposed in the [[CSS3GCPM]] draft (which also matches the [[CSS3-MARQUEE]] proposal). or the 'continue' property as described here.

Fragmented Overflow

When the computed value of 'continue' for an element is ''continue/fragments'', and implementations would otherwise have created a box for the element, then implementations must create a sequence of fragment boxes for that element. (It is possible for an element with ''continue: fragments'' to generate only one fragment box. However, if an element's computed 'continue' is not ''continue/fragments'', then its box is not a fragment box.) Every fragment box is a fragmentation container, and any overflow that would cause that fragmentation container to fragment causes another fragment box created as a next sibling of the previous one. Or is it as though it's a next sibling of the element? Need to figure out exactly how this interacts with other box-level fixup. Additionally, if the fragment box is also a multi-column box (as defined in [[!css-multicol-1]] though it defines multi-column container) any content that would lead to the creation of overflow columns [[!css-multicol-1]] instead is flown into an additional fragment box. However, fragment boxes may themselves be broken (due to fragmentation in a fragmentation context outside of them, such as pages, columns, or other fragment boxes); such breaking leads to fragments of the same fragment box rather than multiple fragment boxes. (This matters because fragment boxes may be styled by their index; such breaking leads to multiple fragments of a fragment box with a single index. This design choice is so that breaking a fragment box across pages does not break the association of indices to particular pieces of content.) Should a forced break that breaks to an outer fragmentation context cause a new fragment of a single fragment box or a new fragment box? Should we find a term other than fragment box here to make this a little less confusing?

What if we want to be able to style the pieces of an element split within another type of fragmentation context? These rules prevent ever using ''::nth-fragment()'' for that, despite that the name seems the most logical name for such a feature.

<!DOCTYPE HTML>
<title>Breaking content into
  equal-sized cards</title>
<style>
  .in-cards {
    continue: fragments;

    width: 13em;
    height: 8em;

    padding: 4px;
    border: medium solid blue;
    margin: 6px;

    font: medium/1.3 Times New
      Roman, Times, serif;
  }
</style>
<div class="in-cards">
  In this example, the text in the div
  is broken into a series of cards.
  These cards all have the same style.
  The presence of enough content to
  overflow one of the cards causes
  another one to be created.  The second
  card is created just like it's the
  next sibling of the first.
</div>

<!DOCTYPE HTML>
<style>
  .article {
    continue: fragments;
  }
  .article::first-letter {
    font-size: 2em;
    line-height: 0.9;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    max-lines: 3;
  }
  .article::nth-fragment(2) {
    column-count: 2;
  }
</style>
<div class="article">
  ...
</div>

We should specify that ''continue: fragments'' does not apply to at least some table parts, and perhaps other elements as well. We need to determine exactly which ones.

This specification needs to say which type of fragmentation context is created so that it's clear which values of the 'break-*' properties cause breaks within this context. We probably want ''break-*: region'' to apply.

This specification needs a processing model that will apply in cases where the layout containing the fragments has characteristics that use the intrinsic size of the fragments to change the amount of space available for them, such as [[CSS3-GRID-LAYOUT]]. There has already been some work on such a processing model in [[CSS-REGIONS-1]], and the work done on a model there, and the editors of that specification, should inform what happens in this specification.

Fragment styling

The ''::nth-fragment()'' pseudo-element

The ::nth-fragment() pseudo-element is a pseudo-element that describes some of the fragment boxes generated by an element. The argument to the pseudo-element takes the same syntax as the argument to the :nth-child() pseudo-class defined in [[!SELECT]], and has the same meaning except that the number is relative to fragment boxes generated by the element instead of siblings of the element.

Selectors that allow addressing fragments by counting from the end rather than the start are intentionally not provided. Such selectors would interfere with determining the number of fragments.

Depending on future discussions, this ''::nth-fragment(an+b)'' syntax may be replaced with the new ''::fragment:nth(an+b)'' syntax.

Styling of fragments

Should this apply to continue:fragments only, or also to continue:paginate? (If it applies, then stricter property restrictions would be needed for continue:paginate.)

In the absence of rules with ''::nth-fragment()'' pseudo-elements, the computed style for each fragment box is the computed style for the element for which the fragment box was created. However, the style for a fragment box is also influenced by rules whose selector's subject [[!SELECT]] has an ''::nth-fragment()'' pseudo-element, if the 1-based number of the fragment box matches that ''::nth-fragment()'' pseudo-element and the selector (excluding the ''::nth-fragment()'' pseudo-element) matches the element generating the fragments.

When determining the style of the fragment box, these rules that match the fragment pseudo-element cascade together with the rules that match the element, with the fragment pseudo-element adding the specificity of a pseudo-class to the specificity calculation. Does this need to be specified in the cascading module as well?

<!DOCTYPE HTML>
<style>
  .bouncy-columns {
    continue: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .bouncy-columns::nth-fragment(1) {
    background: aqua; color: black;
    transform: rotate(-3deg);
  }
  .bouncy-columns::nth-fragment(2) {
    background: yellow; color: black;
    transform: rotate(3deg);
  }
</style>
<div class="bouncy-columns">
  ...
</div>

Styling an ''::nth-fragment()'' pseudo-element with the 'continue' property does take effect; if a fragment box has a computed value of 'continue' other than ''fragments'' then that fragment box is the last fragment. However, overriding 'continue' on the first fragment does not cause the fragment box not to exist; whether there are fragment boxes at all is determined by the computed value of overflow for the element.

Styling an ''::nth-fragment()'' pseudo-element with the 'content' property has no effect; the computed value of 'content' for the fragment box remains the same as the computed value of content for the element.

Specifying ''display: none'' for a fragment box causes the fragment box with that index not to be generated. However, in terms of the indices used for matching ''::nth-fragment()'' pseudo-elements of later fragment boxes, it still counts as though it was generated. However, since it is not generated, it does not contain any content.

Specifying other values of 'display', 'position', or 'float' is permitted, but is not allowed to change the inner display type. (Since 'continue' only applies to block containers, flex containers, and grid containers). Need to specify exactly how this works

To match the model for other pseudo-elements where the pseudo-elements live inside their corresponding element, declarations in ''::nth-fragment()'' pseudo-elements override declarations in rules without the pseudo-element. The relative priority within such declarations is determined by normal cascading order (see [[!CSS2]]).

Styles specified on ''::nth-fragment()'' pseudo-elements do affect inheritance to content within the fragment box. In other words, the content within the fragment box must inherit from the fragment box's style (i.e., the pseudo-element style) rather than directly from the element. This means that elements split between fragment boxes may have different styles for different parts of the element.

This inheritance rule allows specifying styles indirectly (by using explicit ''inherit'' or using default inheritance on properties that don't apply to ''::first-letter'') that can't be specified directly (based on the rules in the next section). This is a problem. The restrictions that apply to styling inside fragments should also apply to inheritance from fragments.

<!DOCTYPE HTML>
<style>
  .article {
    continue: fragments;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    margin-bottom: 1em;
    height: 4em;
  }
  .article::nth-fragment(2) {
    margin-left: 5em;
    margin-right: 2em;
  }
</style>
<div class="article">
  The <code>font-size</code> property...
</div>

Styling inside fragments

Should this apply to continue:fragments only, or also to continue:paginate?

The ''::nth-fragment()'' pseudo-element can also be used to style content inside of a fragment box. Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements, the ''::nth-fragment()'' pseudo-element can be applied to parts of the selector other than the subject: in particular, it can match ancestors of the subject. However, the only CSS properties applied by rules with such selectors are those that apply to the ''::first-letter'' pseudo-element.

To be more precise, when a rule's selector has ''::nth-fragment()'' pseudo-elements attached to parts of the selector other than the subject, the declarations in that rule apply to a fragment (or pseudo-element thereof) when:

1. the declarations are for properties that apply to the ''::first-letter'' pseudo-element,
2. the declarations would apply to that fragment (or pseudo-element thereof) had those ''::nth-fragment()'' pseudo-elements been removed, with a particular association between each sequence of simple selectors and the element it matched, and
3. for each removed ''::nth-fragment()'' pseudo-element, the fragment lives within a fragment box of the element associated in that association with the selector that the pseudo-element was attached to, and whose index matches the pseudo-element.

<!DOCTYPE HTML>
<style>
  .dark-columns {
    continue: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin-right: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .dark-columns::nth-fragment(1) {
    background: aqua; color: black;
  }
  .dark-columns::nth-fragment(1) :link {
    color: blue;
  }
  .dark-columns::nth-fragment(1) :visited {
    color: purple;
  }
  .dark-columns::nth-fragment(2) {
    background: navy; color: white;
  }
  .dark-columns::nth-fragment(2) :link {
    color: aqua;
  }
  .dark-columns::nth-fragment(2) :visited {
    color: fuchsia;
  }
</style>
<div class="dark-columns">
  ...
</div>

Appendix B: Scroll navigation controls

{{HTMLLinkElement/scrollTargetElement}} attribute

Scroll tracking

Focus behavior

Appendix C: Privacy Considerations

Appendix D: Security Considerations

Changes Since Level 4

Acknowledgments

Thanks especially to the feedback from Rossen Atanassov, Bert Bos, Tantek Çelik, John Daggett, fantasai, Daniel Glazman, Vincent Hardy, Håkon Wium Lie, Peter Linss, Robert O'Callahan, Florian Rivoal, Alan Stearns, Steve Zilles, and all the rest of the www-style community.