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 <h1>CSS Overflow Module Level 3</h1>

 <pre class="metadata">

 Status: ED

 Work Status: Exploring

 ED: https://drafts.csswg.org/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, [email protected], 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>

 <!-- FIXME: the break-* link doesn't actually work! -->

 <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 export>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 export>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 export>ink overflow region</dfn> is the non-rectangular area

 	occupied by the <a>ink overflow</a>, and the

 	<dfn export>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 export>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 export>scrollable overflow region</dfn> is the non-rectangular region

 	occupied by the <a>scrollable overflow</a>, and the

 	<dfn export>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>unioned with</em> the border boxes

 		    of all boxes for which it is the containing block

 		    (accounting for transforms as described below).

 		    Issue: Only Blink (and presumably WebKit) use the margin box for abspos; other browsers use the border box.

 		    Can we convince everyone else to change? Because using the border box is kindof hostile.

 		<!-- 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="https://drafts.csswg.org/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]].

 	Issue: Browsers seem to agree on using axis-aligned rectangles (in the element's own coord space)

 	to determine overflow.

 	This means scrollbars can appear when not actually necessary.

 	See <a href="http://software.hixie.ch/utilities/js/live-dom-viewer/?%3C!DOCTYPE%20html%3E%0A%3Cdiv%20class%3Dparent%3E%3Cdiv%20class%3Dchild%3E%3Cdiv%20class%3D%27grandchild%27%3E%3C%2Fdiv%3E%3C%2Fdiv%3E%3C%2Fdiv%3E%0A%3Cstyle%3E%0Adiv%20{%20padding%3A%2030px%3B%20min-width%3A%2030px%3B%20float%3A%20left%3B%20border%3A%20thin%20dotted%3B%20background%3A%20rgba%280%2C0%2C0%2C.2%29%3B}%0A.parent%20{%20overflow%3A%20auto%3B%20background%3A%20rgba%28255%2C0%2C0%2C.2%29%3B%20}%0A.child%20{%20transform%3A%20rotate%28-45deg%29%3B%20padding%3A%203px%3B%20background%3A%20rgba%280%2C255%2C0%2C.2%29%3B%20}%0A.grandchild%20{%20transform%3A%20rotate%2845deg%29%20scale%28.9%29%3B%20background%3A%20rgba%280%2C0%2C255%2C.2%29%3B%20}">testcase</a>

 	(adjust the ''scale(.9)'' to ''scale(.7)'' to see the scrollbars disappear,

 	as the blue grandchild is no longer growing the bounds of the green child).

 	Is this behavior we want to spec,

 	or is it something browsers would like to fix someday?

 <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>

 	These properties specify whether a box’s content

 	(including any <a>ink overflow</a>)

 	is clipped to its padding edge,

 	and if so,

 	whether it is a <dfn export>scrollable box</dfn>

 	that allows the user to scroll clipped parts of its <a>scrollable overflow region</a>

 	into view.

 	<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>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>overflow</a> in the vertical direction

 	(i.e., overflow from the top and bottom sides of the box).

 	<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'.

 	Values have the following meanings:

 	<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 Propagation</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.

 	For HTML UAs,

 	if this would result in the viewport having ''visible'' overflow,

 	the UA must instead apply the 'overflow' property

 	set on the <{body}> element to the viewport.

 	In either case,

 	the used value of 'overflow' for the element from which the value is propagated

 	must evaluate to ''visible''.

 	If this application would result in the viewport having ''visible'' overflow,

 	the UA must instead treat the viewport as having ''overflow/auto'' overflow.

 <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 export>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 by default the <a>block-start</a>/</a>inline-start</a> edge

 	of the box’s <a>padding edge</a>.

 	However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]

 	can be used to change this.

 	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,

 		Håkon 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.