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<pre class='metadata'>
Title: CSS Layout API Level 1
Status: DREAM
Group: houdini
ED: https://drafts.css-houdini.org/css-layout-api-1/
Shortname: css-layout-api
Level: 1
Abstract:
Editor: Greg Whitworth, gwhit@microsoft.com, w3cid 69511
Editor: Ian Kilpatrick, ikilpatrick@chromium.org, w3cid 73001
Editor: Tab Atkins-Bittner, Google, http://xanthir.com/contact/, w3cid 42199
Editor: Shane Stephens, shanestephens@google.com, w3cid 47691
Editor: Robert O'Callahan, robert@ocallahan.org
Editor: Rossen Atanassov, rossen.atanassov@microsoft.com, w3cid 49885
Ignored Terms: LayoutWorklet
Ignored Terms: create a workletglobalscope
</pre>
<style>
/* Put nice boxes around each algorithm. */
[data-algorithm]:not(.heading) {
padding: .5em;
border: thin solid #ddd; border-radius: .5em;
margin: .5em calc(-0.5em - 1px);
}
[data-algorithm]:not(.heading) > :first-child {
margin-top: 0;
}
[data-algorithm]:not(.heading) > :last-child {
margin-bottom: 0;
}
</style>
<pre class="link-defaults">
spec:css-break-3; type:dfn; text:fragment
spec:css-display-3; type:dfn; text:box
spec:css-display-3; type:value; for:display; text:none
spec:css-display-3; type:value; for:<display-inside>; text:grid
spec:css-display-3; type:value; for:<display-outside>; text:inline
spec:css-pseudo-4; type:selector; text:::after
spec:css-pseudo-4; type:selector; text:::before
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:dom; type:dfn; for:/; text:element
spec:infra; type:dfn; text:list
spec:html; type:dfn; for:global object; text:realm
spec:css22; type:property;
text:max-height
text:max-width
text:min-height
text:min-width
</pre>
<pre class="anchors">
urlPrefix: https://heycam.github.io/webidl/; type: dfn;
text: InvalidModificationError
urlPrefix: #dfn-;
url: throw; text: thrown
urlPrefix: #idl-;
text: boolean
text: DOMException
url: es-type-mapping; text: converting
url: es-invoking-callback-functions; text: Invoke
urlPrefix: https://tc39.github.io/ecma262/#sec-; type: dfn;
text: constructor
text: Construct
text: GetMethod
text: IsArray
text: IsCallable
text: IsConstructor
text: HasProperty
url: ecmascript-data-types-and-values; text: type
url: get-o-p; text: Get
url: generatorfunction; text: generator function
url: terms-and-definitions-function; text: function
urlPrefix: native-error-types-used-in-this-standard-
text: TypeError
urlPrefix: https://drafts.csswg.org/css-display-3/#; type: dfn
text: flow layout
urlPrefix: https://drafts.csswg.org/css-sizing/#; type: dfn
text: intrinsic sizes
urlPrefix: https://drafts.csswg.org/css-break/#; type: dfn
text: fragmentation break
urlPrefix: https://www.w3.org/TR/CSS21/; type:dfn
urlPrefix: box.html#;
url: box-dimensions; text: box model edges
urlPrefix: visudet.html#;
text: static position
urlPrefix: https://html.spec.whatwg.org/#; type: dfn
text: structuredserializeforstorage
text: structureddeserialize
</pre>
Introduction {#intro}
=====================
<em>This section is not normative.</em>
The layout stage of CSS is responsible for generating and positioning <a>fragments</a> from the
<a>box tree</a>.
This specification describes an API which allows developers to layout a <a>box</a> in response to
computed style and <a>box tree</a> changes.
Layout API Containers {#layout-api-containers}
==============================================
A new <a href="https://www.w3.org/TR/css-values-3/#comb-one">alternative</a> value is added
to the <<display-inside>> production: <code>layout(<<ident>>)</code>.
<dl dfn-for="display" dfn-type=value>
<dt><dfn>layout()</dfn>
<dd>
This value causes an element to generate a <a>layout API container</a> box.
</dl>
A <dfn>layout API container</dfn> is the box generated by an element with a <<display-inside>>
<a>computed value</a> ''layout()''.
A <a>layout API container</a> establishes a new <dfn>layout API formatting context</dfn> for its
contents. This is the same as establishing a block formatting context, except that the layout
provided by the author is used instead of the block layout.
For example, floats do not intrude into the layout API container, and the layout API container's
margins do not collapse with the margins of its contents.
All inflow children of a <a>layout API container</a> are called <dfn>layout API children</dfn> and
are laid out using the auther defined layout.
<a>Layout API containers</a> form a containing block for their contents
<a href="https://www.w3.org/TR/CSS2/visudet.html#containing-block-details">exactly like block
containers do</a>. [[!CSS21]]
Note: In a future level of the specification there may be a way to override the containing block
behaviour.
The 'overflow' property applies to <a>layout API containers</a>. This is discussed in
[[#interaction-overflow]].
As the layout is entirely up to the author, properties which are used in other layout modes (e.g.
flex, block) may not apply. For example an author may not repect the 'margin' property on children.
Layout API Container Painting {#painting}
-----------------------------------------
<a>Layout API Container</a> children paint exactly the same as inline blocks [[!CSS21]], except that
the order in which they are returned from the layout method (via
{{FragmentResultOptions/childFragments}}) is used in place of raw document order, and 'z-index'
values other than ''z-index/auto'' create a stacking context even if 'position' is ''static''.
Box Tree Transformations {#layout-api-box-tree}
-----------------------------------------------
The <a>layout API children</a> can act in different ways depending on the value of <a for="document
layout definition">layout options'</a> {{LayoutOptions/childDisplay}} (set by
<code>layoutOptions</code> on the class).
If the value of <a for="document layout definition">layout options'</a>
{{LayoutOptions/childDisplay}} is <code>"block"</code> the 'display' value of that child is
<a>blockified</a>. This is similar to children of <a>flex containers</a> or <a>grid containers</a>.
See [[!css3-display]].
If the value of <a for="document layout definition">layout options'</a>
{{LayoutOptions/childDisplay}} is <code>"normal"</code>, no <a>blockification</a> occurs. Instead
children with a <<display-outside>> <a>computed value</a> of ''inline'' (a <a>root inline box</a>)
will produce a single {{LayoutFragment}} representing each line when
{{LayoutChild/layoutNextFragment()}} is called.
Note: This allows authors to adjust the available inline size of each line, and position each line
separately.
Children of a {{LayoutChild}} which represents <a>root inline box</a> also have some additional
transformations.
- A <a>block-level</a> box inside a <a>inline-level</a> box is <a>inlinified</a> I.e. its
<<display-outside>> is set to ''inline''.
- A <a>float</a> inside a <a>inline-level</a> box is not taken out of flow. Instead it must be
treated as inflow, and be <a>inlinified</a>.
In both of the above cases the children become <a>atomic inlines</a>.
Note: User agents would not perform any "inline splitting" or fragmenting when they encounter a
<a>block-level</a> box.
<div class="note">
Note: In the example below "inline-span" would be represented as a single {{LayoutChild}} with
both "block" and "float" being <a>atomic inlines</a>.
<pre class="lang-html">
<span id="inline-span">
Text
<div id="block"></div>
<div id="float"></div>
Text
</span>
</pre>
</div>
Layout API Model and Terminology {#layout-api-model-and-terminology}
====================================================================
This section gives an overview of the Layout API given to authors.
The <dfn>current layout</dfn> is the layout algorithm for the <a>box</a> we are currently performing
layout for.
The <dfn>parent layout</dfn> is the layout algorithm for the <a>box's</a> direct parent, (the layout
algorithm which is requesting the <a>current layout</a> to be performed).
A <dfn>child layout</dfn> is the layout algorithm for a {{LayoutChild}} of the <a>current layout</a>.
Layout Children {#layout-children}
----------------------------------
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutChild {
readonly attribute StylePropertyMapReadOnly styleMap;
IntrinsicSizesRequest intrinsicSizes();
LayoutFragmentRequest layoutNextFragment(LayoutConstraints constraints, ChildBreakToken breakToken);
};
</pre>
The {{LayoutChild}} has internal slot(s):
- <dfn attribute for=LayoutChild>\[[box]]</dfn> a CSS <a>box</a>.
- <dfn attribute for=LayoutChild>\[[styleMap]]</dfn> a {{StylePropertyMapReadOnly}}, this is the
computed style for the child, it is populated with only the properties listed in
<code>childInputProperties</code>.
<hr>
A {{LayoutChild}} represents either a CSS generated <a>box</a> before layout has occured. (The box
or boxes will all have a computed value of 'display' that is not ''none'').
The {{LayoutChild}} does not contain any layout information itself (like inline or block size) but
can be used to generate {{LayoutFragment}}s which do contain layout information.
An author cannot construct a {{LayoutChild}} with this API, this happens at a separate stage of the
rendering engine (post style resolution).
A {{LayoutChild}} could be generated by:
- An <a>element</a>.
- A <a>root inline box</a>.
- A <a>::before</a> or <a>::after</a> pseudo-element.
Note: Other pseudo-elements such as <a>::first-letter</a> or <a>::first-line</a> do not generate
a {{LayoutChild}} for layout purposes. They are additional styling information for a text
node.
- An <a>anonymous box</a>. For example an anonymous box may be inserted as a result of:
- A text node which has undergone <a>blockification</a>. (Or more generally a <a>root inline
box</a> which has undergone <a>blockification</a>).
- An element with ''display: table-cell'' which doesn't have a parent with ''display: table''.
<div class="note">
Note: As an example the following would be placed into three {{LayoutChild}}ren:
<pre class="lang-html">
<style>
#box::before { content: 'hello!'; }
</style>
<!-- A ::before pseudo-element is inserted here. -->
<div id="box">A block level box with text.</div>
<img src="..." />
</pre>
</div>
<div class="note">
Note: As an example the following would be placed into a single {{LayoutChild}} as they share a
<a>root inline box</a>:
<pre class="lang-html">
This is a next node, <span>with some additional styling,
that may</span> break over<br>multiple lines.
</pre>
</div>
Multiple non-<a>atomic inlines</a> are placed within the same {{LayoutChild}} to allow rendering
engines to perform text shaping across element boundaries.
<div class="note">
Note: As an example the following should produce one {{LayoutFragment}} but is from
three non-<a>atomic inlines</a>:
<pre class="lang-html">
ع<span style="color: blue">ع</span>ع
</pre>
</div>
An array of {{LayoutChild}}ren is passed into the layout method which represents the children of the
current box which is being laid out.
<div algorithm>
The <dfn attribute for=LayoutChild>styleMap</dfn>, on getting from a {{LayoutChild}} |this|, the
user agent must perform the following steps:
1. Return |this|' {{StylePropertyMapReadOnly}} contained in the {{[[styleMap]]}} internal slot.
</div>
<div algorithm>
When the <dfn method for=LayoutChild>layoutNextFragment(|constraints|, |breakToken|)</dfn> method is
called on a {{LayoutChild}} |this|, the user agent must perform the following steps:
1. Let |request| be a new {{LayoutFragmentRequest}} with internal slot(s):
- {{LayoutFragmentRequest/[[layoutChild]]}} set to |this|.
- {{[[layoutConstraints]]}} set to |constraints|.
- {{[[breakToken]]}} set to |breakToken|.
2. Return |request|.
</div>
<div algorithm>
When the <dfn method for=LayoutChild>intrinsicSizes()</dfn> method is called on a {{LayoutChild}}
|this|, the user agent must perform the following steps:
1. Let |request| be a new {{IntrinsicSizesRequest}} with internal slot(s):
- {{IntrinsicSizesRequest/[[layoutChild]]}} set to |this|.
2. Return |request|.
</div>
Note: Both {{LayoutChild/layoutNextFragment()}} and {{LayoutChild/intrinsicSizes()}} don't
synchronously run. See [[#request-objects]] for a full description.
### LayoutChildren and the Box Tree ### {#layout-child-box-tree}
Each <a>box</a> has a <dfn attribute for=box>\[[layoutChildMap]]</dfn> internal slot, which is a
<a>map</a> of {{LayoutWorkletGlobalScope}}s to {{LayoutChild}}ren.
<div algorithm="get a layout child">
When the user agent wants to <dfn>get a layout child</dfn> given |workletGlobalScope|, |name|, and
|box|, it <em>must</em> run the following steps:
1. Assert that:
- |box| is currently attached to the <a>box tree</a>.
- |box|'s <a>containing block</a> is a <a>layout API container</a>.
- The <a>containing block</a>'s ''layout()'' function's first argument is |name|.
2. Let |layoutChildMap| be |box|'s {{[[layoutChildMap]]}}.
3. If |layoutChildMap|[|workletGlobalScope|] does not <a for=map>exist</a>, run the following
steps:
1. Let |definition| be the result of <a>get a layout definition</a> given |name|, and
|workletGlobalScope|.
Assert that <a>get a layout definition</a> succeeded, and |definition| is not
<code>"invalid"</code>.
2. Let |childInputProperties| be |definition|'s <a for="layout definition">child input
properties</a>.
3. Let |layoutChild| be a new {{LayoutChild}} with internal slot(s):
- {{[[box]]}} set to |box|.
- {{[[styleMap]]}} set to a new {{StylePropertyMapReadOnly}} populated with
<em>only</em> the <a>computed values</a> for properties listed in
|childInputProperties|.
4. <a for=map>Set</a> |layoutChildMap|[|workletGlobalScope|] to |layoutChild|.
4. Return the result of <a for=map>get</a> |layoutChildMap|[|workletGlobalScope|]
</div>
When a <a>box</a> is inserted into the <a>box tree</a> the user agent <em>may</em> pre-populate the
{{[[layoutChildMap]]}} for all {{LayoutWorkletGlobalScope}}s.
When a <a>box</a> is removed from the <a>box tree</a> the user agent <em>must</em> clear the
{{[[layoutChildMap]]}}.
<div algorithm="update a layout child style">
When the user agent wants to <dfn>update a layout child style</dfn> given |box|, it <em>must</em>
run the following steps:
1. Assert that:
- |box| is currently attached to the <a>box tree</a>.
2. If |box|'s <a>containing block</a> is not a <a>layout API container</a>, abort all these
steps.
3. Let |layoutChildMap| be |box|'s {{[[layoutChildMap]]}}.
4. <a for=map>For each</a> |layoutChild| in |layoutChildMap|:
1. Let |styleMap| be |layoutChild|'s {{[[styleMap]]}}.
2. Update |styleMap|'s <a>declarations</a> based on the |box|'s new computed style.
</div>
When the computed style of a <a>box</a> changes the user agent must run the <a>update a layout child
style</a> algorithm.
Layout Fragments {#layout-fragments}
------------------------------------
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutFragment {
readonly attribute double inlineSize;
readonly attribute double blockSize;
attribute double inlineOffset;
attribute double blockOffset;
readonly attribute any data;
readonly attribute ChildBreakToken? breakToken;
};
</pre>
The {{LayoutFragment}} has internal slot(s):
- <dfn attribute for=LayoutFragment>\[[layoutFragmentRequest]]</dfn> a
{{LayoutFragmentRequest}}, this is the fragment request which generated this fragment.
- <dfn attribute for=LayoutFragment>\[[generator]]</dfn> the generator which produced this
fragment.
<hr>
A {{LayoutFragment}} represents a CSS <a>fragment</a> of a {{LayoutChild}} after layout has occurred
on that child. This is produced by the {{LayoutChild/layoutNextFragment()}} method.
The {{LayoutFragment}} has {{LayoutFragment/inlineSize}} and {{LayoutFragment/blockSize}}
attributes, which are set by the respective child's layout algorithm. They represent the <b>border
box</b> size of the CSS <a>fragment</a>, and are relative to the <a>current layout's</a> writing
mode.
The {{LayoutFragment/inlineSize}} and {{LayoutFragment/blockSize}} attributes cannot be changed. If
the <a>current layout</a> requires a different {{LayoutFragment/inlineSize}} or
{{LayoutFragment/blockSize}} the author must perform {{LayoutChild/layoutNextFragment()}} again with
different arguments in order to get different results.
The author inside the current layout can position a resulting {{LayoutFragment}} by setting its
{{LayoutFragment/inlineOffset}} and {{LayoutFragment/blockOffset}} attributes. If not set by the
author they default to zero. The {{LayoutFragment/inlineOffset}} and {{LayoutFragment/blockOffset}}
attributes represent the position of the {{LayoutFragment}} relative to its parent's <b>border
box</b>, before transform or positioning (e.g. if a fragment is <a>relatively positioned</a>) has
been applied.
<div class="example">
The layout algorithm performs a block-like layout (positioning fragments sequentially in the block
direction), while centering its children in the inline direction.
<pre class="lang-javascript">
registerLayout('block-like', class {
*intrinsicSizes(children, edges, styleMap) {
const childrenSizes = yield children.map((child) => {
return child.intrinsicSizes();
});
const maxContentSize = childrenSizes.reduce((max, childSizes) => {
return Math.max(max, childSizes.maxContentSize);
}, 0) + edges.all.inline;
const minContentSize = childrenSizes.reduce((max, childSizes) => {
return Math.max(max, childSizes.minContentSize);
}, 0) + edges.all.inline;
return {maxContentSize, minContentSize};
}
*layout(children, edges, constraints, styleMap) {
const availableInlineSize = constraints.fixedInlineSize - edges.all.inline;
const availableBlockSize = (constraints.fixedBlockSize || Infinity) - edges.all.block;
const childFragments = [];
const childConstraints = { availableInlineSize, availableBlockSize };
const childFragments = yield children.map((child) => {
return child.layoutNextFragment(childConstraints);
});
let blockOffset = edges.all.blockStart;
for (let fragment of childFragments) {
// Position the fragment in a block like manner, centering it in the
// inline direction.
fragment.blockOffset = blockOffset;
fragment.inlineOffset = Math.max(
edges.all.inlineStart,
(availableInlineSize - fragment.inlineSize) / 2);
blockOffset += fragment.blockSize;
}
const autoBlockSize = blockOffset + edges.all.blockEnd;
return {
autoBlockSize,
childFragments,
};
}
});
</pre>
</div>
A <a>layout API container</a> can communicate with other <a>layout API containers</a> by using the
{{LayoutFragment/data}} attribute. This is set by the {{FragmentResultOptions/data}} member in the
{{FragmentResultOptions}} dictionary.
The {{LayoutFragment}}'s {{LayoutFragment/breakToken}} specifies where the {{LayoutChild}} last
fragmented. If the {{LayoutFragment/breakToken}} is null the {{LayoutChild}} wont produce any more
{{LayoutFragment}}s for that token chain. The {{LayoutFragment/breakToken}} can be passed to the
{{LayoutChild/layoutNextFragment()}} function to produce the next {{LayoutFragment}} for a
particular child. The {{LayoutFragment/breakToken}} cannot be changed.
If the <a>current layout</a> requires a different {{LayoutFragment/breakToken}} the author must perform
{{LayoutChild/layoutNextFragment()}} again with different arguments.
<div algorithm>
When the user agent wants to <dfn>create a layout fragment</dfn> given |generator|,
|layoutFragmentRequest|, and |internalFragment|, it <em>must</em> run the following steps:
1. Let |targetRealm| be |generator|'s <a>Realm</a>.
2. Let |fragment| be a new {{LayoutFragment}} with:
- The {{[[layoutFragmentRequest]]}} internal slot being |layoutFragmentRequest|.
- The {{[[generator]]}} internal slot being |generator|.
- {{LayoutFragment/inlineSize}} being |internalFragment|'s <a>inline size</a> relative to
the <a>current layout's</a> writing mode.
- {{LayoutFragment/blockSize}} being |internalFragment|'s <a>block size</a> relative to the
<a>current layout's</a> writing mode.
- {{LayoutFragment/inlineOffset}} initially set to 0.
- {{LayoutFragment/blockOffset}} initially set to 0.
- {{LayoutFragment/breakToken}} being a new {{ChildBreakToken}} representing
|internalFragment|'s internal break token, if any.
- If |internalFragment| has a |clonedData| object stored with it, let
{{LayoutFragment/data}} being the result of
<a>StructuredDeserialize</a>(|clonedData|, |targetRealm|), otherwise null.
3. Return |fragment|.
</div>
Intrinsic Sizes {#intrinsic-sizes}
----------------------------------
<pre class='idl'>
[Exposed=LayoutWorklet]
interface IntrinsicSizes {
readonly attribute double minContentSize;
readonly attribute double maxContentSize;
};
</pre>
The {{IntrinsicSizes}} object has internal slot(s):
- <dfn attribute for=IntrinsicSizes>\[[intrinsicSizesRequest]]</dfn> a
{{IntrinsicSizesRequest}}, this is the intrinsic sizes request which generated these
intrinsic sizes.
<hr>
A {{IntrinsicSizes}} object represents the <a>min-content size</a> and <a>max-content size</a> of a
CSS <a>box</a>. It has {{IntrinsicSizes/minContentSize}} and {{IntrinsicSizes/maxContentSize}}
attributes which represent the <b>border box</b> min/max-content contribution of the {{LayoutChild}}
for the <a>current layout</a>. The attributes are relative to the inline direction of the <a>current
layout's</a> writing mode.
The {{IntrinsicSizes/minContentSize}} and {{IntrinsicSizes/maxContentSize}} cannot be changed. They
must not change for a {{LayoutChild}} within the current layout pass.
<div class="example">
The example below shows the border-box intrinsic sizes of two children.
<pre class="lang-html">
<style>
.child-0 {
width: 380px;
border: solid 10px;
}
.child-1 {
border: solid 5px;
}
.box {
display: layout(intrinsic-sizes-example);
font: 25px/1 Ahem;
}
</style>
<div class="box">
<div class="child-0"></div>
<div class="child-1">XXX XXXX</div>
</div>
</pre>
<pre class="lang-javascript">
registerLayout('intrinsic-sizes-example', class {
*intrinsicSizes(children, edges, styleMap) {
const childrenSizes = yield children.map((child) => {
return child.intrinsicSizes();
});
childrenSizes[0].minContentSize; // 400, (380+10+10) child has a fixed size.
childrenSizes[0].maxContentSize; // 400, (380+10+10) child has a fixed size.
childrenSizes[1].minContentSize; // 100, size of "XXXX".
childrenSizes[1].maxContentSize; // 200, size of "XXX XXXX".
}
*layout() {}
});
</pre>
</div>
<div algorithm>
When the user agent wants to <dfn>create an intrinsic sizes object</dfn> given
|intrinsicSizesRequest|, and |internalIntrinsicSizes|, it <em>must</em> run the following steps:
1. Let |intrinsicSizes| be a new {{IntrinsicSizes}} with:
- The {{[[intrinsicSizesRequest]]}} internal slot being |intrinsicSizesRequest|.
- {{IntrinsicSizes/minContentSize}} being |internalIntrinsicSizes|' <b>border box</b>
min-content contribution, relative to the <a>current layout's</a> writing mode.
- {{IntrinsicSizes/maxContentSize}} being |internalIntrinsicSizes|'s <b>border box</b>
max-content contribution, relative to the <a>current layout's</a> writing mode.
2. Return |intrinsicSizes|.
</div>
Layout Constraints {#layout-constraints}
----------------------------------------
<pre class='idl'>
[Constructor(optional LayoutConstraintsOptions options),Exposed=LayoutWorklet]
interface LayoutConstraints {
readonly attribute double availableInlineSize;
readonly attribute double availableBlockSize;
readonly attribute double? fixedInlineSize;
readonly attribute double? fixedBlockSize;
readonly attribute double percentageInlineSize;
readonly attribute double percentageBlockSize;
readonly attribute double? blockFragmentationOffset;
readonly attribute BlockFragmentationType blockFragmentationType;
readonly attribute any data;
};
dictionary LayoutConstraintsOptions {
double availableInlineSize = 0;
double availableBlockSize = 0;
double fixedInlineSize;
double fixedBlockSize;
double percentageInlineSize;
double percentageBlockSize;
double blockFragmentationOffset;
BlockFragmentationType blockFragmentationType = "none";
any data;
};
enum BlockFragmentationType { "none", "page", "column", "region" };
</pre>
A {{LayoutConstraints}} object is passed into the layout method which represents the all the
constraints for the <a>current layout</a> to perform layout inside. It is also used to pass
information about the available space into a <a>child layout</a>.
The {{LayoutConstraints}} object has {{LayoutConstraints/availableInlineSize}} and
{{LayoutConstraints/availableBlockSize}} attributes. This represents the <a>available space</a> for
a {{LayoutFragment}} which the layout should respect.
Note: Some layouts may need to produce a {{LayoutFragment}} which exceed this size. For example a
<a>replaced element</a>. The <a>parent layout</a> should expect this to occur and deal with it
appropriately.
A <a>parent layout</a> may require the <a>current layout</a> to be exactly a particular size. If
the {{LayoutConstraints/fixedInlineSize}} or {{LayoutConstraints/fixedBlockSize}} are specified the
<a>current layout</a> should produce a {{LayoutFragment}} with a the specified size in the
appropriate direction.
<div class="example">
The layout algorithm performs a flexbox-like distribution of spare space in the inline direction. It
creates child layout constraints which specify that a child should be a fixed inline size.
<pre class="lang-javascript">
registerLayout('flex-distribution-like', class {
*intrinsicSizes(children, edges, styleMap) {
const childrenSizes = yield children.map((child) => {
return child.intrinsicSizes();
});
const maxContentSize = childrenSizes.reduce((sum, childSizes) => {
return sum + childSizes.maxContentSize;
}, 0) + edges.all.inline;
const minContentSize = childrenSizes.reduce((max, childSizes) => {
return sum + childSizes.minContentSize;
}, 0) + edges.all.inline;
return {maxContentSize, minContentSize};
}
*layout(children, edges, constraints, styleMap) {
const availableInlineSize =
constraints.fixedInlineSize - edges.all.inline;
const availableBlockSize =
(constraints.fixedInlineSize || Infinity) - edges.all.block;
const childConstraints = { availableInlineSize, availableBlockSize };
const unconstrainedChildFragments = yield children.map((child) => {
return child.layoutNextFragment(childConstraints);
});
const unconstrainedSizes = [];
const totalSize = unconstrainedChildFragments.reduce((sum, fragment, i) => {
unconstrainedSizes[i] = fragment.inlineSize;
return sum + fragment.inlineSize;
}, 0);
// Distribute spare space between children.
const remainingSpace = Math.max(0, inlineSize - totalSize);
const extraSpace = remainingSpace / children.length;
const childFragments = yield children.map((child, i) => {
return child.layoutNextFragment({
fixedInlineSize: unconstrainedSizes[i] + extraSpace,
availableBlockSize
});
});
// Position the fragments.
let inlineOffset = 0;
let maxChildBlockSize = 0;
for (let fragment of childFragments) {
fragment.inlineOffset = inlineOffset;
fragment.blockOffset = edges.all.blockStart;
inlineOffset += fragment.inlineSize;
maxChildBlockSize = Math.max(maxChildBlockSize, fragment.blockSize);
}
return {
autoBlockSize: maxChildBlockSize + edges.all.block,
childFragments,
};
}
});
</pre>
</div>
The {{LayoutConstraints}} object has {{LayoutConstraints/percentageInlineSize}} and
{{LayoutConstraints/percentageBlockSize}} attributes. These represent the size that a layout
percentages should be resolved against while performing layout.
The {{LayoutConstraints}} has a {{LayoutConstraints/blockFragmentationType}} attribute. The
<a>current layout</a> should produce a {{LayoutFragment}} which fragments at the
{{LayoutConstraints/blockFragmentationOffset}} if possible.
The <a>current layout</a> can choose not to fragment a {{LayoutChild}} based on the
{{LayoutConstraints/blockFragmentationType}}, for example if the child has a property like
''break-inside: avoid-page;''.
<div algorithm>
When the user agent wants to <dfn>create a layout constraints object</dfn> given |sizingMode|, box|,
and |internalLayoutConstraints|, it <em>must</em> run the following steps:
1. If |sizingMode| is <code>"block-like"</code> then:
1. Let |fixedInlineSize| be the result of calculating |box|'s <b>border-box</b>
<a>inline size</a> (relative to |box|'s writing mode) exactly like block containers do.
2. Let |fixedBlockSize| be null if |box|'s <a>block size</a> is ''height/auto'', otherwise
the result of calculating |box|'s <b>border-box</b> <a>block size</a> exactly like block
containers do.
3. Return a new {{LayoutConstraints}} object with:
- {{LayoutConstraints/fixedInlineSize}}, and {{LayoutConstraints/availableInlineSize}}
set to |fixedInlineSize|.
- {{LayoutConstraints/percentageInlineSize}} set to |internalLayoutConstraints|'
percentage resolution size in the inline axis (relative to |box|'s writing mode).
- {{LayoutConstraints/fixedBlockSize}} set to |fixedBlockSize|.
- {{LayoutConstraints/availableBlockSize}} set to |fixedBlockSize| if not null,
otherwise |internalLayoutConstraints|' <a>available space</a> in the block axis
(relative to |box|'s writing mode).
- {{LayoutConstraints/percentageBlockSize}} set to |internalLayoutConstraints|'
percentage resolution size in the block axis (relative to |box|'s writing mode).
2. If |sizingMode| is <code>"manual"</code> then:
1. Return a new {{LayoutConstraints}} object with:
- {{LayoutConstraints/fixedInlineSize}}/{{LayoutConstraints/fixedBlockSize}} set to
|internalLayoutConstraints|' fixed inline/block size (relative to |box|'s writing
mode) imposed by the <a>parent layout</a>. Either may be null.
Note: See [[#interaction-sizing]] for different scenarios when this can occur.
- {{LayoutConstraints/availableInlineSize}}/{{LayoutConstraints/availableBlockSize}} set
to |internalLayoutConstraints|' <a>available space</a>.
- {{LayoutConstraints/percentageInlineSize}}/{{LayoutConstraints/percentageBlockSize}}
set to |internalLayoutConstraints|' percentage resolution size.
</div>
Breaking and Fragmentation {#breaking-and-fragmentation}
--------------------------------------------------------
<pre class="idl">
[Exposed=LayoutWorklet]
interface ChildBreakToken {
readonly attribute BreakType breakType;
readonly attribute LayoutChild child;
};
[Exposed=LayoutWorklet]
interface BreakToken {
readonly attribute sequence<ChildBreakToken> childBreakTokens;
readonly attribute any data;
};
dictionary BreakTokenOptions {
sequence<ChildBreakToken> childBreakTokens;
any data = null;
};
enum BreakType { "none", "line", "column", "page", "region" };
</pre>
A {{LayoutChild}} can produce multiple {{LayoutFragment}}s. A {{LayoutChild}} may fragment in the
block direction if a {{LayoutConstraints/blockFragmentationType}} is not none. Additionally
{{LayoutChild}} which represents <a>inline-level</a> content, may fragment line by line if the
<a for="document layout definition">layout options'</a> {{LayoutOptions/childDisplay}} (set by
<code>layoutOptions</code>) is <code>"normal"</code>.
A subsequent {{LayoutFragment}} is produced by using the previous {{LayoutFragment}}'s
{{LayoutFragment/breakToken}}. This tells the <a>child layout</a> to produce a {{LayoutFragment}}
starting at the point encoded in the {{ChildBreakToken}}.
<div class="example">
This example shows a simple layout which indents child fragments for a certain number of
lines.
This example also demonstrates using the previous {{LayoutFragment/breakToken}} of a
{{LayoutFragment}} to produce the next fragment for the {{LayoutChild}}.
It also demonstrates using the {{BreakToken}} to respect the {{LayoutConstraints}}'
{{LayoutConstraints/blockFragmentationType}}, it resumes it layout from the previous {{BreakToken}}.
It returns a {{FragmentResultOptions}} with a {{FragmentResultOptions/breakToken}} which is used to
resume the layout.
<pre class="lang-javascript">
registerLayout('indent-lines', class {
static layoutOptions = {childDisplay: 'normal'};
static inputProperties = ['--indent', '--indent-lines'];
*layout(children, edges, constraints, styleMap, breakToken) {
// Determine our (inner) available size.
const availableInlineSize =
constraints.fixedInlineSize - edges.all.inline;
const availableBlockSize =
(constraints.fixedBlockSize || Infinity) - edges.all.block;
// Detrermine the number of lines to indent, and the indent amount.
const indent = resolveLength(constraints, styleMap.get('--indent'));
let lines = styleMap.get('--indent-lines').value;
const childFragments = [];
let childBreakToken = null;
if (breakToken) {
childBreakToken = breakToken.childBreakTokens[0];
// Remove all the children we have already produced fragments for.
children.splice(0, children.indexOf(childBreakToken.child));
}
let blockOffset = edges.all.blockStart;
let child = children.shift();
while (child) {
const shouldIndent = lines-- > 0;
// Adjust the inline size for the indent.
const childAvailableInlineSize = shouldIndent ?
availableInlineSize - indent : availableInlineSize;
const childConstraints = {
availableInlineSize: childAvailableInlineSize,
availableBlockSize,
percentageInlineSize: availableInlineSize,
blockFragmentationType: constraints.blockFragmentationType,
};
const fragment = yield child.layoutNextFragment(childConstraints,
childBreakToken);
childFragments.push(fragment);
// Position the fragment.
fragment.inlineOffset = shouldIndent ?
edges.all.inlineStart + indent : edges.all.inlineStart;
fragment.blockOffset = blockOffset;
blockOffset += fragment.blockSize;
// Check if we have gone over the block fragmentation limit.
if (constraints.blockFragmentationType != 'none' &&
blockOffset > constraints.blockSize) {
break;
}
if (fragment.breakToken) {
childBreakToken = fragment.breakToken;
} else {
// If a fragment doesn't have a break token, we move onto the
// next child.
child = children.shift();
childBreakToken = null;
}
}
const autoBlockSize = blockOffset + edges.all.blockEnd;
// Return our fragment.
const result = {
autoBlockSize,
childFragments: childFragments,
}
if (childBreakToken) {
result.breakToken = {
childBreakTokens: [childBreakToken],
};
}
return result;
}
});
</pre>
</div>
Edges {#edges}
--------------
<pre class='idl'>
[Exposed=LayoutWorklet]
interface LayoutEdgeSizes {
readonly attribute double inlineStart;
readonly attribute double inlineEnd;
readonly attribute double blockStart;
readonly attribute double blockEnd;
// Convenience attributes for the sum in one direction.
readonly attribute double inline;
readonly attribute double block;
};
[Exposed=LayoutWorklet]
interface LayoutEdges {
readonly attribute LayoutEdgeSizes border;
readonly attribute LayoutEdgeSizes scrollbar;
readonly attribute LayoutEdgeSizes padding;
readonly attribute LayoutEdgeSizes all;
};
</pre>
A {{LayoutEdges}} object is passed into the layout method. This represents the size of the <a>box
model edges</a> for the current box which is being laid out.
The {{LayoutEdges}} has {{LayoutEdges/border}}, {{LayoutEdges/scrollbar}}, and
{{LayoutEdges/padding}} attributes. Each of these represent the width of their respective edge.
The {{LayoutEdges}} has the {{LayoutEdges/all}} attribute. This is a convenience attribute which
represents the sum of the {{LayoutEdges/border}}, {{LayoutEdges/scrollbar}}, {{LayoutEdges/padding}}
edges.
The {{LayoutEdgeSizes}} object represents the width in CSS pixels of an edge in each of the
<a>abstract dimensions</a> ({{LayoutEdgeSizes/inlineStart}}, {{LayoutEdgeSizes/inlineEnd}},
{{LayoutEdgeSizes/blockStart}}, {{LayoutEdgeSizes/blockEnd}}).
The {{LayoutEdgeSizes/inline}}, and {{LayoutEdgeSizes/block}} on the {{LayoutEdgeSizes}} object are
convenience attributes which represent the sum in that direction.
<div class="example">
This example shows an node styled by CSS, and what its respective {{LayoutEdges}} could contain.
<pre class="lang-html">
<style>
.container {
width: 50px;