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<pre class='metadata'>
Title: CSS Grid Layout Module Level 3
Shortname: css-grid
Level: 3
Status: ED
Work Status: Exploring
Group: csswg
TR: https://www.w3.org/TR/css-grid-3/
ED: https://drafts.csswg.org/css-grid-3/
Editor: Tab Atkins Jr., Google, http://www.xanthir.com/contact/, w3cid 42199
Editor: Elika J. Etemad / fantasai, Apple, http://fantasai.inkedblade.net/contact, w3cid 35400
Former Editor: Mats Palmgren, Mozilla, mailto:mats@mozilla.com
Editor: Jen Simmons, Apple, http://jensimmons.com/, w3cid 52801
Editor: Brandon Stewart, Apple, https://brandonstewart.net, w3cid 138640
Abstract: This module introduces masonry layout as an additional layout mode for <a href="https://www.w3.org/TR/css-grid-2/">CSS Grid</a> containers.
WPT Path Prefix: css/css-grid/masonry/tentative/
WPT Display: open
Markup Shorthands: css yes
Status Text: <strong>This specification represents two variations on the proposal for masonry layout. Feedback on the alternatives is welcome.</strong>
Ignored Terms: display, used value, computed value, repeat(), content box, containing block, margin boxes,
Ignore MDN Failure: #tracks-alignment, #masonry-auto-flow
</pre>
<pre class=link-defaults>
spec: css-align-3;
type: value; for: align-content;
text: center;
text: stretch;
type: dfn; text: alignment baseline;
spec: css-cascade-5
type: dfn; text: shorthand property;
spec: css-grid-2; type: dfn;
text: auto-placement algorithm
text: computed track list
text: collapsed track
spec: css-flexbox-1;
type:property;
text: order
type:dfn;
text: order-modified document order
</pre>
<style>
:is(section, p).option {
border-right: 1em double;
padding-right: 1em;
}
:is(section, p).option.grid {
border-right-color: #be2596;
}
:is(section, p).option.masonry {
border-right-color: #96be25;
}
:is(span, a).option {
font-weight: bold;
}
:is(span, a).option * {
color: inherit;
}
:is(span, a).option.grid {
color: #be2596;
}
:is(span, a).option.masonry {
color: #658118
}
</style>
<h2 id='intro'>
Introduction</h2>
<em>This section is not normative.</em>
Grid Layout is a layout model for CSS
that has powerful abilities to control the sizing and positioning
of boxes and their contents.
Grid Layout is optimized for 2-dimensional layouts:
those in which alignment of content is desired in both dimensions.
<figure>
<img src="images/grid-layout.png"
alt="An example of grid layout:
two rows of items,
the first being four items — the last of which spans both rows,
and the second being two items —
the first of which spans the first two columns —
plus the spanned item from the first row.">
<figcaption>Representative Grid layout example</figcaption>
</figure>
Although many layouts can be expressed with regular Grid Layout,
restricting items into a grid in both axes also makes it impossible
to express some common layouts on the Web.
This module defines a layout system that removes that restriction
so that items can be placed into Grid-like tracks in just one of the axes,
while stacking them one after another in the other axis.
Items are placed into the column (or row) with the most remaining space
based on the layout size of the items placed so far.
This module also extends <a href="https://www.w3.org/TR/css-grid-2/">CSS Grid</a>
with this new grid item placement strategy
and <a href="https://drafts.csswg.org/css-align">CSS Box Alignment</a> with new alignment features.
<h3 id='background'>
Background and Motivation</h3>
<h4 id='waterfall'>
Waterfall Layout with Auto-placed Items</h4>
[=Masonry layout=], sometimes also called “waterfall layout”,
is a common Web design pattern where a number of items--
commonly images or short article summaries--
are placed one by one into columns
in a way that loosely resembles stone masonry.
Unlike [[CSS-MULTICOL-1|multi-column layout]],
where content is placed vertically in the first column
until it must spills over to the second column,
[=masonry layout=] selects a column for each new item
such that it is generally closer to the top of the layout than items placed later.
<div class="example">
The Pinterest search results page exemplifies this layout:
<figure>
<img src="images/pinterest.png"
alt="An example of masonry layout:
four columns of items,
each item is placed into the column with the smallest height so far.">
<figcaption>Representative masonry layout example</figcaption>
</figure>
Here, each item has a different height
(depending on the content and the width of the column),
and inspecting the DOM reveals
(as the visual content itself gives no indication of ordering)
that each item has been placed into the column with the smallest height so far.
</div>
This layout superficially looks similar to multi-column layout;
but it has the advantage that scrolling down
will naturally lead to "later" items in the layout
(that is, those less relevant in the search results).
It's not possible to achieve this layout using earlier CSS layout models,
unless you know up-front how tall each item will be,
or use JavaScript for content measurement or placement.
Using a [=masonry container=] together with ''grid-area/auto''-positioned items
yields this type of masonry layout.
<h4 id='collapse'>
One-dimensional Grid Layout</h4>
[=Grid layout=] allows for powerful track sizing and explicit placement in two axes,
but sometimes a layout only needs alignment of its items in one dimension.
Using a [=masonry container=] together with explicitly-positioned items
allows for this type of one-dimensional grid layout.
<div class="example">
This example <a href="https://github.com/w3c/csswg-drafts/issues/10233#issuecomment-2071279204">by Douglas Graham</a>
uses explicit positioning to place each item into its assigned column;
but there are no rows.
Instead, items in each column stack one after the other.
This layout can't be duplicated in [=grid layout=]
because the “spanning” relationships among the items in adjacent columns
is not fixed: it depends on their relative heights
and whether the optional banner or advertisement items are included.
It also can't be duplicated in [=flex layout=]
because the source order of the items
(which is used for reading, sequential navigation, and one-column mobile phone layout)
goes back and forth between the two columns.
<figure>
<!-- https://codepen.io/TabAtkins/pen/abeoBOm -->
<img src="images/masonry-page-layout.png"
alt="In one-column layout:
the header, followed by an optional banner, the secondary navigation,
the main content area, an advertisement block, and finally the footer.
In two-column layout:
the header spanning both columns on top,
the footer spanning both columns at the bottom,
in the wider left column the optional banner followed by the main content area,
and in the narrow left column the secondary navigation followed by the advertisement block.">
<figcaption>
Comparison of one-column and two-column variants of a one-dimensional grid layout.
</figcaption>
</figure>
</div>
<h3 id="values">
Value Definitions</h3>
This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]].
Combination with other CSS modules may expand the definitions of these value types.
In addition to the property-specific values listed in their definitions,
all properties defined in this specification
also accept the <a>CSS-wide keywords</a> as their property value.
For readability they have not been repeated explicitly.
<h2 id='masonry-model'>
Masonry Layout Model</h2>
<dfn>Masonry layout</dfn>
lays out items into pre-defined tracks similar to [=grid layout=] in one axis
(called the <dfn local-lt="grid-axis">grid axis</dfn>),
but flows them freely similar to [=flex layout=] in the other
(called the <dfn local-lt="stacking-axis">stacking axis</dfn>).
Similar to [=grid layout=] and unlike [=flex layout=],
[=masonry layout=]’s auto-placement
distributes items across the tracks to keep the lengths of those tracks
as similar as possible.
[=Grid items=] are formed and [=blockified=]
exactly the same as in a regular [=grid container=].
(For clarity, [=grid items=] and [=grid tracks=] of a [=masonry container=]
can be referred to as <dfn export lt="masonry item">masonry items</dfn>
and <dfn export lt="masonry track">masonry tracks</dfn>.)
All CSS properties work the same as in a regular [=grid container=]
unless otherwise specified by this specification.
For example, 'order' can be used to specify a different layout order for the items.
Note: Subgrid items are supported,
but subgridding only occurs in the [=grid axis=];
see [[#subgrids]] for details.
A <dfn>masonry container</dfn> is a box whose contents participate in [=masonry layout=].
A [=masonry container=] is a <dfn export>column masonry container</dfn>
if its [=stacking axis=] is the [=block axis=],
or a <dfn export>row masonry container</dfn>
if its [=stacking axis=] is the [=inline axis=].
<table class=data>
<caption>Comparing Masonry Containers</caption>
<tr>
<th style="writing-mode: vertical-rl; writing-mode: sideways-lr">
Column Masonry
<td>
<pre>
grid-template-columns: 1fr 2fr 3fr;
</pre>
<td>
<img src="images/masonry-columns.png"
alt="Column masonry lays out items in columns,
but ordered across the columns,
placing each item in the then-shortest column.">
<tr>
<th style="writing-mode: vertical-rl; writing-mode: sideways-lr">
Row Masonry
<td>
<pre>
grid-template-rows: 1fr 2fr 3fr;
</pre>
<td>
<img src="images/masonry-rows.png"
alt="Row masonry lays out items in rows,
but ordered down across the rows,
placing each item in the then-shortest row.">
</table>
<h3 id="order-accessibility">
Reordering and Accessibility</h3>
Although [=masonry layout=] generally progresses in a forwards fashion
(placing the next item down or endward of the current item,
matching the natural "reading order"),
it can switch between these two in a seemingly arbitrary manner.
In simple cases, the 'item-slack' property can help reduce
the feeling of backtracking due to small sizing differences in the [=block axis=]
when laying out auto-placed items.
But when [[#masonry-layout-algorithm|auto-placement]] is mixed with [=definite position|explicit placement=] or spanning items,
some amount of backtracking may occur.
<div class=example>
For example, in the following markup sample,
the fourth item is a spanner that doesn't fit
in the remaining empty column on the first line.
It ends up positioned into the into the first column,
which is the highest available space into which it will fit.
The next few items, which have a span of 1,
end up laying out “above” it in the empty column,
violating the natural reading order.
<xmp highlight=html>
<section class=masonry>
<div class=item>1</div>
<div class=item>2</div>
<div class="item tall">3</div>
<div class="item wide">4</div>
<div class=item>5</div>
<div class=item>6</div>
<div class=item>7</div>
</section>
<style>
.masonry {
//FIXME display:masonry or something
grid-template-columns: repeat(5, auto);
}
.item { height: 50px; }
.item.wide { grid-column: span 3; }
.item.tall { height: 90px; }
</style>
</xmp>
<figure>
<img src="images/masonry-reorder-span.png"
alt="In this example, the first row is items 1, 2, 3, 5, 6,
with item 3 slightly taller than the others.
Item 4 spans the first three columns, and is placed
just below item 3, while item 7 is tucked under item 5.">
<figcaption>Auto-placed masonry layout with mixed-height items and mixed span sizes</figcaption>
</figure>
Similarly, items explicitly placed into specific tracks can leave gaps behind them,
into which subsequent auto-placed items can be placed visually out-of-order.
</div>
Authors should be aware of these possibilities
and design layouts where such backtracking is minimized
so that focus and reading order can be more easily followed.
Alternatively, if the items do not have an inherent order,
use the 'reading-flow' property to allow the UA to re-order the items
for reading and linear navigation.
ISSUE: Or should reordering be the default behavior for auto-placed items here?
<div class=note>
Techniques for reducing backtracking include:
* Using appropriate values for 'item-slack',
i.e. values large enough to avoid gratuitous differentiation among similarly-sized tracks,
but not so large that meaningful differences get ignored.
* Using explicit placement in ways that help group related items together,
rather than ways that disrupt the natural order of items
* Avoiding the combination of mixed span sizes in the [=grid axis=]
and disparate item sizes in the [=stacking axis=],
which can cause items to get pulled out of order (see example above).
</div>
As with [=grid layout=] and [=flex layout=]
authors can use the 'order' property to re-order items;
the same caveats apply.
See [[css-grid-2#order-accessibility]]
and [[css-flexbox-1#order-accessibility]].
<h3 id="masonry-switch">
Establishing Masonry Layout</h3>
<div class=issue>
ISSUE(11593): We are still debating what syntactic switch establishes a [=masonry layout=].
Proposals so far include:
* ''display: masonry'' (or a synonym, such as ''display: stack'')
* ''item-pack: collapse'' (see 'item-pack')
* a <css>collapse</css> value for 'grid-template-rows'/'grid-template-columns',
indicating that axis as the [=stacking axis=] of a [=grid container=]
</div>
<h2 id="masonry-track-templates">
Masonry Track Specification</h2>
In the [=grid axis=],
the full power of [=grid layout=] is available for track specification:
* Track sizes, line names, and areas can be specified on the [=masonry container=]’s [=grid axis=],
just like in [=grid layout=].
* The [=explicit grid=] and [=implicit grid=] are formed in the same way as for a regular [=grid container=].
* Items can be [[#masonry-track-placement|placed]] against these grid templates just as in [=grid layout=].
However,
auto-placed items contribute sizing to all tracks,
not just the track into which they are ultimately placed;
see [[#track-sizing]].
Note: This is because auto-placed items must be laid out <em>as</em> they are placed,
so that each track knows how “full” it is
(and therefore which track should receive the next auto-placed item);
thus, the tracks themselves must already have a definite size
so that the items know their [=available space=] during layout.
<h3 id="masonry-track-grid-option">
Declaring Masonry Track Templates: the 'grid-template-*' properties</h3>
The 'grid-template-*' and 'grid-auto-rows'/'grid-auto-columns' properties
(and their shorthands)
apply in the [=grid axis=] of the [=masonry container=]
and establish tracks just as on regular [=grid containers=].
(They are ignored in the [=stacking axis=].)
ISSUE(10869): What should be the initial track listing?
<h4 id=masonry-intrinsic-repeat>
Intrinsic Tracks and repeat()</h4>
ISSUE(10915): Should we allow auto-repeated content-based tracks?
Is this a reasonable definition for them?
Should they work also in Grid Layout somehow?
In Grid Layout,
all [=grid items=] are placed in the grid
<em>before</em> the grid tracks are sized.
This implies that ''repeat()/auto-fill''/''repeat()/auto-fit'' repetition
can't include intrinsically sized tracks such as ''grid-template-rows/auto''
(either in the ''repeat()'' function <em>or</em> alongside it
in the fixed portion of the track list),
as that would require the layout algorithm
to have already determined which items would go in those tracks,
to determine how large the tracks are,
to determine how many repetitions fit in the available space.
In Masonry Layout,
as [=masonry item=] placement and layout are intertwined and somewhat simplified,
this restriction is no longer strictly required.
It requires a slightly heuristic definition of sizing,
but auto repetition <em>can</em> include intrinsically-sized tracks
in a [=masonry container=]
(and the initial value of 'masonry-template-tracks' uses this!).
<div algorithm="determine intrinsic repetitions">
To determine the number of repetitions
that a ''repeat()'' function resolves to,
run layout as defined in [[#track-sizing-performance]],
with the following changes:
* Expand ''repeat()/auto-fill''/''repeat()/auto-fit'' repeat functions once.
* Ignore item placement.
(That is, assume all items have ''masonry-track: auto''.)
* If a [=masonry item=] has a span larger than 1,
then for each of its intrinsic sizes
that it would contribute to the [=virtual masonry item=],
first subtract the combined size of the gaps it would span,
and divide by its span.
Then treat it as being a span-1 item with those sizes.
Any intrinsically-sized tracks are then treated as having the size
calculated by this simplified layout
(including those in ''repeat()'' arguments,
taking from their corresponding single repetition)
for the purpose of determining how many repetitions
the ''repeat()'' functions resolve to.
</div>
<details class=note>
<summary>Motivation</summary>
This simplified layout heuristic is defined to be "good enough",
while remaining fast and consistent.
Ignoring placement is required just to make the concept coherent;
before you know how many repetitions you need,
you can't tell what track an item with a definite placement
will end up in.
By chopping spanning items into span-1 items,
this avoids the possible need to expand a repeat() multiple times,
and the incoherent possibility of getting different sizes
for the same keyword across the repetitions.
It also makes the layout as a whole significantly cheaper,
as you only need to consider each unique track size;
you don't even <em>really</em> need to do any repeat() expansion.
That is, in ''auto repeat(auto-fill, min-content auto)'',
both of the ''grid-template-rows/auto'' keywords
will resolve to the same size under this heuristic layout;
you can just figure out what
a ''masonry-template-tracks: auto''
and ''masonry-template-tracks: min-content''
would each result in,
and use those sizes.
</details>
<h3 id="subgrids">
Subgrids</h3>
[=Subgridding=] allows nested [=masonry containers=] (and [=grid containers=])
to share track sizes.
If the parent's corresponding axis is a [=grid axis=],
the subgridded axis is taken from the parent container
[[css-grid-2#subgrids|as specified for grid containers]];
if the parent's corresponding axis is a [=stacking axis=],
the subgridded axis acts like ''masonry''.
Note: If this results in ''grid-template/masonry'' in both axes,
it is resolved as normal for [=masonry containers=] with double-axis ''grid-template/masonry'' templates,
i.e. it acts like ''grid-template-columns: none; grid-template-rows: masonry''.
In [=masonry layout=], auto-placed [=subgrids=]
don't inherit any line names from their parent grid,
because that would make the placement of the item
dependent on layout results;
but the subgrid's tracks are still aligned to the parent's tracks as usual.
<div class="example">
Here's a subgrid <a href="examples/subgrid-example-1.html">example</a>:
```css
<style>
.grid {
//FIXME: display: inline-grid; or something
grid-template-rows: auto auto 100px;
align-content: center;
height: 300px;
border: 1px solid;
}
.grid > * {
margin: 5px;
background: silver;
}
.grid > :nth-child(2n) {
background: pink;
}
.grid subgrid {
display: grid;
grid: subgrid / subgrid;
grid-row: 2 / span 2;
grid-gap: 30px;
}
.grid subgrid > * { background: cyan; }
</style>
```
```html
<div class="grid">
<item>1</item>
<item>2</item>
<item>3</item>
<subgrid>
<item style="height:100px">subgrid.1</item>
<item>sub.2</item>
<item>s.3</item>
</subgrid>
<item>4</item>
<item>5</item>
<item style="width: 80px">6</item>
<item>7</item>
</div>
```
<figure>
<img src="images/subgrid-example-1.png">
<figcaption>
The rendering of the subgrid example above.
</figcaption>
</figure>
Note how the subgrid's first item ("subgrid.1") contributes
to the intrinsic size of the 2nd row in the parent grid.
This is possible since the subgrid specified a definite position
so we know which tracks it will occupy.
Note also that trying to subgrid the parent's [=stacking axis=]
results in the subgrid getting [=masonry layout=] in its [=inline axis=].
</div>
A [=subgrid=] that is a [=masonry container=]
can be referred to as a <dfn export>submasonry</dfn>.
<h3 id="repeat-notation">
Track Repetition: the ''repeat()'' notation</h3>
This specification introduces new keywords and masonry-specific behavior
for the ''repeat()'' notation.
<h4 id="repeat-auto-areas">
repeat(auto-areas)</h4>
The new <dfn value for="repeat()">auto-areas</dfn> value for the ''repeat()'' notation
represents the number of repetitions necessary
for the total number of explicit tracks to match
the 'grid-template-areas' / 'masonry-template-areas' value
in effect in the corresponding axis.
If multiple tracks are listed for the repetition,
the final repetition is truncated as necessary
to produce the correct number of tracks.
Note: Unlike ''repeat()/auto-fit''--
which always repeats at least once and always repeats the track listing entirely--
the number of repetitions for ''repeat()/auto-areas'' can be zero
(if there are already enough explicit tracks),
and the final repetition can be partial.
If 'grid-template-areas' / 'masonry-template-areas' is ''grid-template-areas/none'',
this value behaves as ''auto-fit''.
Note: This value applies both to regular [=grid containers=] and to [=masonry containers=].
ISSUE(10854): It's unclear if we actually need this value.
Note that the explicit grid already takes values from 'grid-auto-columns'/'grid-auto-rows'/'masonry-auto-tracks'
as needed to match the number of template areas.
<h4 id="repeat-auto-fit">
repeat(auto-fit)</h4>
In [=masonry containers=] (as in regular [=grid containers=])
''repeat()/auto-fit'' acts like ''repeat()/auto-fill'',
but with empty tracks [=collapsed tracks|collapsed=].
However, because placement occurs after track sizing,
[=masonry containers=] use a heuristic
to determine if a track will be occupied:
* All tracks occupied by explicitly placed items are considered occupied.
* With the sum of the spans of all auto-placed items as <var>N</var>,
all unoccupied tracks up to the <var>N</var>th such track
are considered occupied.
All tracks produced by the ''repeat()/auto-fit'' repetition and considered unoccupied by this heuristic
are assumed “empty” and are [=collapsed tracks|collapsed=].
A [=collapsed track=] cannot accept placement of auto-placed items.
Note: It is possible for an auto-placed item to be placed in a track when ''repeat()/auto-fill'' is used
that would be collapsed if ''repeat()/auto-fit'' is used
if there are auto-placed items with a span greater than 1
mixed with explicitly-placed items that leave gaps too small for the auto-placed items.
<h3 id="track-sizing">
Grid Axis Track Sizing</h3>
Track sizing works the same as in [[css-grid-2#algo-track-sizing|CSS Grid]],
except that when considering which items contribute to intrinsic sizes:
* All items explicitly placed in that track contribute, and
* All items with an [=automatic grid position=] contribute
(regardless of whether they are ultimately placed in that track).
<div class="example">
For example, suppose there are two columns in the [=grid axis=]
and that
* Items A, B, and C have no explicit position.
* Item D is explicitly placed into the first column.
In this case, items A, B, C, and D all contribute to sizing the first column,
while only A, B, and C (and not D) contribute to the second column.
</div>
In the case of spanning items with an [=automatic grid position=],
they are assumed to be placed at every possible start position,
and contribute accordingly.
<div class="example">
For example, suppose there are 5 columns in the [=grid axis=],
with the middle having a fixed size of ''100px''
and the other two being ''grid-template/auto''-sized.
For the purpose of track sizing,
an item that spans 2 tracks
and has an intrinsic contribution of 220px
is essentially copied and assumed to exist:
* At grid line 1,
contributing 110px to each of the first two tracks.
* At grid line 2,
contributing 120px to the second track.
* At grid line 3,
contributing 120px to the fourth track.
* At grid line 4,
contributing 110px to the fourth and fifth tracks.
</div>
Note: This algorithm ensures that each track is at least big enough
to accommodate every item that is ultimately placed in it,
and does not create dependency cycles between placement and track sizing.
However, depending on the variation in sizes,
tracks could be larger than necessary:
an exact fit is only guaranteed if
all items are explicitly placed in the [=grid axis=]
or all items are the same size
(or matching multiples of that size, in the case of spanning items).
<h4 id="track-sizing-subgrid">
Subgrid Item Contributions</h4>
When sizing the tracks of either a regular [=grid container=] or a [=masonry container=],
a [=submasonry=] has special handling of items that have an [=automatic grid position=]:
* Any such item is placed into every possible grid track
that could be spanned by the [=submasonry=].
(If the submasonry has a [=definite grid position=], thus only the spanned tracks;
if it has an [=automatic grid position=], then all tracks in the parent grid.)
* Any such item receives the largest margin/border/padding contribution
of each edge at which it could hypothetically be placed.
If the item spans the entire subgrid, it receives both.
(See <a href="https://www.w3.org/TR/css-grid-2/#subgrid-item-contribution">CSS Grid Layout §9</a>.)
<h4 id="track-sizing-performance">
Optimized Track Sizing</h4>
Track sizing can be optimized by aggregating items
that have the same span size and placement
into a single virtual item
as follows:
<ol>
<li>
Separate all the [=masonry items=] into <dfn>item groups</dfn>, according to the following properties:
* the span of the item
* the placement of the item,
i.e. which tracks it is allowed to be placed in
* the item's [=baseline-sharing group=]
Note: For example, an item with span 2 placed in the second track
will be in a different group than an item with span 2 that has an [=automatic grid position=].
<li>
For each [=item group=], synthesize a <dfn>virtual masonry item</dfn>
that has the maximum of every intrinsic size contribution
among the items in that group.
If the items apply [=baseline alignment=],
determine the baselines of the [=virtual masonry item=]
by placing all of its items into a single hypothetical grid track
and finding their shared baseline(s) and shims.
Increase the group's intrinsic size contributions accordingly.
<li>
Place hypothetical copies of each [=virtual masonry item=] into the [=grid axis=] tracks
in every position that the item could potentially occupy,
and run the [[css-grid-2#algo-track-sizing|track sizing algorithm]] with those items.
The resulting track sizes are the [=masonry container's=] track sizes.
</ol>
Note: This optimization should give the same results
as the track sizing description [[#track-sizing|above]];
if not this is an error, please
<a href="https://github.com/w3c/csswg-drafts/issues">report it to the CSSWG</a>.
<h2 id="masonry-track-placement">
Masonry Placement</h2>
In the [=grid axis=],
items can be <em>explicitly placed</em> into tracks and span them using the familiar [=grid-placement properties=]’ syntax.
Auto-placement, however, uses the [[#masonry-layout-algorithm]],
placing each item with an [=automatic grid position=]
into the “shortest” masonry track available.
<div class="example">
Here's a masonry layout <a href="examples/pinterest-with-span.html">example</a>
demonstrating placed and spanning items:
<figure>
<img src="images/example-pinterest-with-span.png">
<figcaption>Rendering of the example above.</figcaption>
</figure>
ISSUE: Need a better example!!!
</div>
<h3 id="masonry-placement-grid-option">
Specifying Masonry Item Placement: the 'grid-column-*' and 'grid-row-*' properties</h3>
The 'grid-column-*' and 'grid-row-*' properties
(and their shorthands)
apply in the [=grid axis=] of the items
and establish placement just as in regular [=grid layout=].
<h3 id="item-slack">
Placement Precision: the 'item-slack' property</h3>
<pre class=propdef>
Name: item-slack
Value: <<length-percentage>> | infinite
Initial: 1em
Percentages: relative to the [=grid-axis=] [=content box=] size of the [=masonry container=]
Inherited: no
Applies to: [=masonry containers=]
Computed value: a computed <<length-percentage>> value
Animation type: as length
</pre>
[=Masonry containers=] are filled
by placing each [=masonry item=]
in whichever [=masonry track=] is currently the least filled.
When multiple tracks are tied for least-filled,
placing the items in order looks good.
But if tracks are only <em>very slightly</em> different heights,
it can look strange to have them not fill in order,
as the height differences aren't perceived as <em>meaningfully</em> different.
The 'item-slack' property specifies what the threshold is
for considering tracks to be “the same height”,
causing them to fill in order.
<dl dfn-type=value dfn-for=item-slack>
: <dfn><<length-percentage>></dfn>
:: Specifies the <dfn dfn for=masonry>tie threshold</dfn>
for the [=masonry container=].
Placement positions are considered to be equally good (“tied”)
if they are within the specified distance
from the shortest position.
Note: The initial value is a “small” distance (''1em'')
that is probably appropriate to represent “close enough”.
: <dfn>infinite</dfn>
:: Specifies an infinite [=tie threshold=].
This makes items distribute themselves strictly in order,
without considering the length of the tracks at all.
Note: This value can result in consecutive items being placed
in dramatically different positions in the [=stacking axis=],
which can be confusing to readers.
If the initial value (`1em`) is too small,
consider a larger value (such as `10em` or `50vh`)
instead of `infinite`.
</dl>
Issue: Is ''1em'' the right default?
<h3 id="masonry-layout-algorithm">
Masonry Layout and Placement Algorithm</h3>
For each of the tracks in the [=grid axis=],
keep a <dfn>running position</dfn> initialized to zero.
Maintain also a <dfn>auto-placement cursor</dfn>,
initially pointing to the first line.
For each item in [=order-modified document order=]:
<ol>
<li>
If the item has a [=definite grid position=] in the [=grid axis=],
use that placement.
ISSUE: Should this also update the placement cursor?
Otherwise, resolve its [=grid axis=] placement using these substeps:
<ol>
<li>Starting at the first [=grid axis=] line in the [=implicit grid=],
find the largest [=running position=] of the [=grid axis=] tracks
that the item would span if it were placed at this line,
and call this position <var>max_pos</var>.
<li>Repeat the previous step for each successive line number
until the item would no longer fit inside the grid.
<li>Let |possible lines| be the line that resulted in the smallest <var>max_pos</var>,
and all lines that result in a <var>max_pos</var> within the [=tie threshold=] of this <var>max_pos</var>.
<li>Choose the first line in |possible lines| greater than or equal to the [=auto-placement cursor=]
as the item's position in the [=grid axis=];
or if there are none such, choose the first one.
<li>Update the [=auto-placement cursor=] to point to item's last line.
</ol>
<li>
Place the item in its [=grid axis=] tracks
at the maximum of the [=running position=]s
of the tracks it spans.
<li>
Calculate the size of the item's <a href="#containing-block">containing block</a>
and then layout the item.
Set the [=running position=] of the spanned [=grid axis=] tracks
to <code><var>max_pos</var> + [=outer size=] + 'grid-gap'</code>.
</ol>
Note: This algorithm chooses the track
that would result in the item being placed as highly as possible.
If there are ties, it chooses the earliest such track,
<em>after</em> the most recently placed item if possible
(ensuring that it always “moves forward” even in the presence of ties).
<h4 id="containing-block">
Containing Block</h4>
The [=containing block=] for a [=grid item=] participating in [=masonry layout=]
is formed by its [=grid area=] in the [=grid axis=]
and the [=grid container=]'s [=content box=] in the [=stacking axis=].
<h4 id="rtl-example">
Placement and Writing Modes</h4>
Note: Like all of [=grid layout=],
masonry layout and placement is sensitive to the [=writing mode=].
For example, for ''direction: rtl'',
items are placed right-to-left rather than left-to-right,
whether the inline axis is a [=grid axis=] or a [=stacking axis=].
<div class="example">
Here's a simple <a href="examples/rtl-grid-axis.html">example</a> using ''direction: rtl'' in the [=grid axis=]:
```css
<style>
.grid {
//FIXME display: inline-grid; or something
direction: rtl;
grid-template-columns: repeat(4, 2ch);
border: 1px solid;
}
item { background: silver }
item:nth-child(2n+1) {
background: pink;
height: 4em;
}
</style>
```
```html
<div class="grid">
<item>1</item>
<item style="grid-column:span 2">2</item>
<item>3</item>
<item>4</item>
</div>
```
<figure>
<img src="images/rtl-grid-axis.png">
<figcaption>Rendering of the ''direction: rtl'' example above.</figcaption>
</figure>
</div>
<div class="example">
Here's a simple <a href="examples/rtl-masonry-axis.html">example</a>
using ''direction: rtl'' in the [=stacking axis=]:
```css
<style>
.grid {
//FIXME display: inline-grid; or something
direction: rtl;
width: 10ch;
column-gap: 1ch;
grid-template-rows: repeat(4, 2em);
border: 1px solid;
}
item { background: silver }
item:nth-child(2n+1) {
background: pink;
width: 4ch;
}
</style>
```
```html
<div class="grid">
<item>1</item>
<item style="grid-row:span 2">2</item>
<item>3</item>
<item>4</item>
</div>
```
<figure>
<img src="images/rtl-masonry-axis.png">
<figcaption>Rendering of the ''direction: rtl'' example above.</figcaption>
</figure>
</div>
<h2 id="intrinsic-sizes">
Sizing Grid Containers</h2>
[[css-grid-2#intrinsic-sizes|Sizing Grid Containers]] works the same as for regular [=grid containers=]
but with the following addendum for the [=stacking axis=]:
The <a>max-content size</a> (<a>min-content size</a>) of a [=grid container=] in the [=stacking axis=]
is the size of the [=masonry box=] in that axis
when sized under a <a>max-content constraint</a> (<a>min-content constraint</a>).
<div class="example">
Here's a simple <a href="examples/grid-intrinsic-sizing-example-1.html">example</a>:
```css
<style>
.grid {
//FIXME display: inline-grid; or something
grid-template-columns: 50px 100px auto;
grid-gap: 10px;
border: 1px solid;
}
item { background: silver; margin: 5px; }
</style>
```
```html
<div class="grid">
<item style="border:10px solid">1</item>
<item>2</item>
<item>3</item>
<item style="height:50px">4</item>
<item>5</item>
<item>6</item>
</div>
```
<figure>
<img src="images/grid-intrinsic-sizing-example-1.png">
<figcaption>Rendering of the [=grid container=] intrinsic sizing example above.</figcaption>
</figure>
</div>
<h2 id="alignment">
Alignment and Spacing</h2>
[[css-grid-2#gutters|Gutters]] are supported in both axes.
In the [=stacking axis=],