Overview.bs

<h1>CSS Overflow Module Level 3</h1>
<pre class="metadata">
Status: ED
Work Status: Exploring
ED: http://dev.w3.org/csswg/css-overflow/
Shortname: css-overflow
Group: csswg
Level: 3
TR: http://www.w3.org/TR/css-overflow-3/
Previous version: http://www.w3.org/TR/2013/WD-css-overflow-3-20130418/
Editor: L. David Baron, Mozilla, http://dbaron.org/
Editor: Florian Rivoal, Invited Expert, florian@rivoal.net, http://florian.rivoal.net
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
!Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.bs">from 27 January 2015 to the present</a>
!Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css-overflow/Overview.src.html">from 28 March 2013 to 27 January 2015</a>
!Change Log: <a href="https://hg.csswg.org/drafts/log/tip/css3-overflow/Overview.src.html">from 31 July 2012 to 27 March 2013</a>
Ignored Vars: B, P
Ignored Terms: padding edge
</pre>
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<pre class="link-defaults">
spec:css-transforms-1; type:property; text:transform-style
type: dfn; spec:css-multicol-1; text:overflow column
</pre>
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<pre class="anchors">
url: http://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
</pre>

<h2 id="intro">
Introduction</h2>

	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 [[CSS21]] 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 specification introduces the long-standing de-facto 'overflow-x' and 'overflow-y' properties,
	and defines overflow handling more fully.

	[Something something 'max-lines'.]

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<h2 id="overflow-concepts">
Types of overflow</h2>

	CSS uses the term <dfn>overflow</dfn> to describe
	the contents of a box
	that extend outside one of that box's edges
	(i.e., its <i>content edge</i>, <i>padding edge</i>,
	<i>border edge</i>, or <i>margin edge</i>).
	The term might be interpreted as
	elements or features that cause this overflow,
	the non-rectangular region occupied by these features,
	or, more commonly,
	the minimal rectangle that bounds that region.
	A box's overflow is computed based on the layout and styling of the box itself
	and of all descendants whose <a>containing block chain</a>
	includes the box.

	In most cases, <a>overflow</a>
	can be computed for any box
	from the bounds and properties of that box itself,
	plus the <a>overflow</a>
	of each of its children.
	However, this is not always the case; for example,
	when ''transform-style: preserve-3d'' [[CSS3-TRANSFORMS]] is used on
	some of the children, any of their descendants with
	''transform-style: preserve-3d'' must also be examined.

	There are three different types of overflow,
	which are used for different purposes by the UA:

	<ul>
		<li><a>ink overflow</a>
		<li><a>scrollable overflow</a>
		<li><a>border-box overflow</a>
	</ul>

<h3 id="ink">Ink overflow</h3>

	The <dfn>ink overflow</dfn> of a box
	is the part of that box and its contents that
	creates a visual effect outside of
	the box's border box.
	Ink overflow is the overflow of painting effects defined to not affect layout
	or otherwise extend the <a>scrollable overflow region</a>,
	such as <a href="http://www.w3.org/TR/css-backgrounds/#box-shadow">box shadows</a>,
	<a href="">border images</a>,
	<a href="https://www.w3.org/TR/css-text-decor-3/">text decoration</a>,
	overhanging glyphs (with negative side bearings,
	or with ascenders/descenders extending outside the em box),
	<a href="https://www.w3.org/TR/CSS2/ui.html#dynamic-outlines">outlines</a>,
	etc.

	Issue: What about 'hanging-punctuation'?

	Since some effects in CSS (for example, the blurs in
	'text-shadow' [[CSS3TEXT]] and 'box-shadow' [[CSS3BG]])
	do not define what visual extent they cover, the extent
	of the <a>ink overflow</a> is undefined.

	The <dfn>ink overflow region</dfn> is the non-rectangular area
	occupied by the <a>ink overflow</a>, and the
	<dfn>ink overflow rectangle</dfn> is
	the minimal rectangle whose axis is aligned to the box's axes
	and contains the <a>ink overflow region</a>.
	Note that the <a>ink overflow rectangle</a> is a rectangle
	in the box's coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].


<h3 id="scrollable">
Scrollable overflow</h3>

	The <dfn>scrollable overflow</dfn> of a box is the
	set of things extending outside of that box's padding edge
	for which a scrolling mechanism needs to be provided.

	The <dfn>scrollable overflow region</dfn> is the non-rectangular region
	occupied by the <a>scrollable overflow</a>, and the
	<dfn>scrollable overflow rectangle</dfn> is
	the minimal rectangle whose axis is aligned to the box's axes
	and contains the <a>scrollable overflow region</a>.

	The <a>scrollable overflow region</a> is the union of:
	<ul>
		<li>the box’s own content and padding areas
		<li>all line boxes directly contained by the box
		<li>the margin boxes <em>and</em> border boxes
		    of all in-flow and floated boxes
		    for which it is the containing block
		    (accounting for transforms as described below).
		<li>the padding boxes of all absolutely-positioned boxes
		    for which it is the containing block
		    (accounting for transforms as described below).

		    Issue: Blink (and presumably WebKit) use the margin box here. Is that better?
		<!-- testcase: http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cstyle%3E%0A%20%20div%20%7B%20height%3A%2050px%3B%20overflow%3A%20scroll%3B%20border%3A%20solid%3B%20position%3A%20relative%3B%7D%0A%20%20p%20%7B%20margin%3A%2025px%3B%20background%3A%20red%3B%20%7D%0A%20%20q%20%7B%20position%3A%20absolute%3B%20background%3A%20blue%3B%20top%3A%2025px%3B%20margin%3A%2025px%3B%20%7D%0A%3C%2Fstyle%3E%0A%3Cdiv%3E%0A%20%20%3Cp%3ETest%20%3Cq%3ETest2%0A%3C%2Fdiv%3E -->

		<li>the <a>scrollable overflow regions</a> of all of the above boxes
		    (accounting for transforms as described below),
		    provided they themselves have ''overflow: visible''
		    (i.e. do not themselves trap the overflow)
		    and that overflow is not already clipped
		    by the 'clip' or 'clip-path' properties.

		    Issue: What about masking?
	</ul>

	<p class="issue">
		The following definition should be rewritten to use
		the concept of <a href="http://dev.w3.org/csswg/css-transforms/#3d-rendering-context">3D rendering context</a> [[!CSS3-TRANSFORMS]]
		and related terms,
		particularly once those concepts stabilize following changes
		proposed in the CSS WG meeting on the morning of 2014-01-28.

	Given the following definitions
	<span class="issue">which belong in [[CSS3-TRANSFORMS]]</span>:

	<dl>
		<dt><dfn>3d-preserving child</dfn>
		<dd>
			A child box <var>B</var> of a containing block <var>C</var>
			is a 3d-preserving child
			if it has ''transform-style: preserve-3d''
			and the user-agent is not required to flatten it
			based on the <a href="http://www.w3.org/TR/css3-transforms/#transform-style-property">requirements</a> in [[!CSS3-TRANSFORMS]].
		
		<dt><dfn>non-3d-preserving child</dfn>
		<dd>
			A child <var>C</var> of a box <var>P</var> is a non-3d-preserving-child
			if it is not a <a>3d-preserving child</a>.

		<dt><dfn>3d-preserving descendant</dfn>
		<dd>
			Box <var>D</var> is a 3d-preserving descendant of box <var>A</var>
			if <var>A</var> is an ancestor of D,
			and <var>D</var> and all of the boxes (if any)
			in the <a>containing block chain</a> from <var>D</var> to <var>A</var>
			are <a>3d-preserving child</a> boxes.
	</dl>

	The <a>scrollable overflow</a> of a box is the union of the following things,
	all adjusted for transforms <span class="issue">undefined concept!</span> into the box's coordinate space:

	<ul>
		<li>
			for the box and all of its <a>3d-preserving descendant</a> boxes:
			<ul>
				<li>the box's own padding edge (for the box itself) or border edge (for <a>3d-preserving descendant</a> boxes)</li>
				<li>the bounds <span class="issue">undefined term!</span> of any text directly in the box</li>
				<li><span class="issue">MORE HERE!</span>
			</ul>
		<li>
			for all the <a>non-3d-preserving child</a> boxes of the
			box and its <a>3d-preserving descendant</a> boxes,
			the scrollable overflow of the box
		</li>
	</ul>

	<p class="issue">
		I wrote this definition off the top of my head,
		so it can't possibly be right.
		It's missing tons of pieces!

	<p class="issue">
		The handling of preserve-3d subtrees here is probably wrong;
		the elements should probably count
		only towards the overflow of the element that flattens them.

	Note that the <a>scrollable overflow rectangle</a> is a always rectangle
	in the box's own coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].

<h3 id="border-box">
Border box overflow</h3>

	<p class="issue">
		This concept has been proposed for some uses, such as for
		determining what the 'outline' property goes around, and
		as the basis of a coordinate system for specifying clips and masks,
		but it's not clear if it's needed.

	The <dfn>border-box overflow</dfn> of a box is the
	union of the box's border edge and the border edges of
	the box's descendants.

	<p class="issue">
		If needed, define more formally, as for scrollable overflow above.
		(Maybe even share the definitions in an appropriate way!)

	The <dfn>border-box overflow region</dfn> is the non-rectangular region
	occupied by the <a>border-box overflow</a>, and the
	<dfn>border-box overflow rectangle</dfn> is
	the minimal rectangle whose axis is aligned to the box's axes
	and contains the <a>border-box overflow region</a>.
	Note that the <a>border-box overflow rectangle</a> is a rectangle
	in the box's coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].


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<h2 id="overflow-properties">
Scrollable Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties</h2>

	<pre class=propdef>
		Name: overflow-x, overflow-y
		Value: ''visible'' | ''hidden'' | ''clip'' | ''scroll'' | ''auto''
		Initial: ''visible''
		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
		Inherited: no
		Percentages: N/A
		Media: visual
		Computed value: see below
		Animatable: no
	</pre>

	The 'overflow-x' property specifies
	the handling of <a>scrollable overflow</a> in the horizontal direction
	(i.e., overflow from the left and right sides of the box),
	and the 'overflow-y' property specifies the handling
	of <a>scrollable overflow</a> in the vertical direction
	(i.e., overflow from the top and bottom sides of the box).
	These properties thereby determine whether or not the box
	is a <dfn>scrollable box</dfn>.

	<pre class=propdef>
		Name: overflow
		Value: ''visible'' | ''hidden'' | ''clip'' | ''scroll'' | ''auto''
		Initial: see individual properties
		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
		Inherited: no
		Percentages: N/A
		Media: visual
		Computed value: see individual properties
		Animatable: no
		Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
	</pre>

	The 'overflow' property is a shorthand property
	that sets the specified values of both 'overflow-x' and 'overflow-y'
	to the value specified for 'overflow'.

	The values of these properties are:

	<dl dfn-for="overflow, overflow-x, overflow-y" dfn-type="value">
		<dt><dfn>visible</dfn>
		<dd>
			There is no special handling of overflow, that is, it
			may be rendered outside the box.
			The box is not a <a>scrollable box</a>.

		<dt><dfn>hidden</dfn>
		<dd>
			This value indicates that
			the box’s content is clipped to its padding box
			and that no scrolling user interface should be provided by the UA
			to view the content outside the clipping region.
			However, the content may still be scrolled programatically,
			for example using the mechanisms defined in [[CSSOM-VIEW]],
			and the box is therefore still a <a>scrollable box</a>.

		<dt><dfn>clip</dfn>
		<dd>
			Like ''hidden'',
			this value indicates that
			the box’s content is clipped to its padding box
			and that no scrolling user interface should be provided by the UA
			to view the content outside the clipping region.
			In addition, unlike ''overflow: hidden''
			which still allows programmatic scrolling,
			''overflow: clip'' forbids scrolling entirely,
			through any mechanism,
			and therefore the box is not a <a>scrollable box</a>.

			Issue: Mozilla implements -moz-hidden-unscrollable,
			which is similar to ''clip'',
			except that it does not cause the element to establish a BFC.
			Should we match that?

		<dt><dfn>scroll</dfn>
		<dd>
			This value indicates that
			the content is clipped to the padding box,
			but can be scrolled into view
			(and therefore the box is a <a>scrollable box</a>).
			Furthermore, if the user agent uses a scrolling mechanism
			that is visible on the screen (such as a scroll bar or a panner),
			that mechanism should be displayed
			whether or not any of its content is clipped.
			This avoids any problem with scrollbars appearing
			and disappearing in a dynamic environment.
			When this value is specified and the target medium is ''print'',
			overflowing content may be printed.

		<dt><dfn>auto</dfn>
		<dd>
			This value indicates that
			the box’s content is clipped to the padding box,
			but can be scrolled into view;
			and therefore the box is a <a>scrollable box</a>.
			However, if the user agent uses a scrolling mechanism
			that is visible on the screen (such as a scroll bar or a panner),
			that mechanism should only be displayed
			if there is overflow.
		</dl>

	If the computed value of 'overflow' on a <a>block box</a> is not ''overflow/visible'',
	it creates a new <a>block formatting context</a> for its content.

<h3 id="overflow-computed-values">
Overflow Value Computation and Propgation</h3>

		<p>The computed values of 'overflow-x' and 'overflow-y'
		are determined from the <a>specified values</a> [[!CSS3CASCADE]]
		based on the following rules:

		<ol>
			<li>
				If one <a>specified value</a> is ''overflow/visible''
				and the other is not,
				then computed values are the <a>specified values</a>
				with ''overflow/visible'' changed to ''overflow/auto''.
			</li>
			<li>
				Otherwise, if both <a>specified values</a> are ''overflow/visible''
				and the computed value of 'contain' is
				one that activates <a>paint containment</a>
				(e.g. ''contain:strict''
				or ''contain: paint''
				or ''contain: layout paint''…),
				then the computed values of both 'overflow-x' and 'overflow-y'
				are changed to ''overflow/clip''.</li>
			<li>
				Otherwise, the computed values are as specified.
			</li>
		</ol>
	</div>

	UAs must apply the 'overflow' property
	set on the root element to the viewport.
	HTML UAs must instead apply the ‘overflow’ property
	from the <{body}> element to the viewport
	if the value on the root element is ''visible''.
	The ''visible'' value when used for the viewport
	must be interpreted as ''overflow/auto''.
	The element from which the value is propagated
	must have a used value for 'overflow' of ''visible''.

<h3 id="scrollbar-layout">
Scrollbars and Layout</h3>

	In the case of a scrollbar being placed on an edge of the element's box,
	it should be inserted between the inner border edge
	and the outer padding edge.
	Any space taken up by the scrollbars should be
	taken out of (subtracted from the dimensions of)
	the containing block formed by the element with the scrollbars.

	Issue: import examples from [[CSS3-BOX]].

<h3 id="scrolling-direction">
Scrolling Origin, Direction, and Restriction</h3>

	The <dfn>initial scroll position</dfn>,
	that is, the initial position of
	the box’s <a>scrollable overflow region</a>
	with respect to its border box,
	prior to any user or programmatic scrolling that changes it,
	is dependent on the box’s <a>writing mode</a>,
	and is the <a>block-start</a>/</a>inline-start</a> edge
	of the box’s <a>padding edge</a>.

	Due to Web-compatibility constraints
	(caused by authors exploiting legacy bugs to surreptitiously hide content from visual readers but not search engines and/or speech output),
	UAs must clip the <a>scrollable overflow region</a>
	of <a>scrollable boxes</a>
	on the <a>block-start</a> and </a>inline-start</a> sides of the box
	(thereby behaving as if they had no <a>scrollable overflow</a> on that side).

	The viewport uses the <a>principal writing mode</a> for these calculations.

	<p class="issue">
		[[CSS3-MARQUEE]] describes an 'overflow-style' property,
		but it has not picked up implementation experience
		that the working group is aware of.
		Should this document treat 'overflow-style' as a defunct proposal,
		or should this document describe the 'overflow-style' property
		and attempt to revive it,
		despite that implementations have implemented
		'overflow-x' and 'overflow-y' instead?


<h2 id="max-lines">
Limiting Number of Visible Text Lines: the 'max-lines' property</h2>


	<pre class=propdef>
		Name: max-lines
		Value: none | <<integer>>
		Initial: none
		Applies to: all non-inline elements
		Inherited: no
		Animatable: as integer
		Percentages: N/A
		Media: visual
		Computed value: specified value
	</pre>

	Issue: Add back 'max-lines', so we can kill the unspecified WebKit feature that does this poorly.


<h2 id="static-media">
Overflow in static media</h2>

	<p class="issue">
		This specification should define useful behavior
		for all values of 'overflow'
		in static media (such as print).
		Current implementation behavior is quite poor and
		produces unexpected results when authors have not considered
		what will happen when
		the content they produce for interactive media
		is printed.


Privacy and Security Considerations {#priv-sec}
===============================================

This specification introduces no new privacy or security concerns.

<h2 class=no-num id="acknowledgments">
Acknowledgments</h2>

	<p>
		Thanks especially to the feedback from
		Rossen Atanassov,
		Bert Bos,
		Tantek Çelik,
		John Daggett,
		fantasai,
		Daniel Glazman,
		Vincent Hardy,
		H&aring;kon Wium Lie,
		Peter Linss,
		Robert O'Callahan,
		Florian Rivoal,
		Alan Stearns,
		Steve Zilles,
		and all the rest of the
		<a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.