Overview.bs

<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: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2013/WD-css-overflow-3-20130418/
Editor: L. David Baron, Mozilla, http://dbaron.org/
Editor: Florian Rivoal, On behalf of Bloomberg, 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
type: dfn; spec:css-transforms-1; text:3d rendering context
</pre>
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<pre class="anchors">
url: https://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 two different types of overflow,
	which are used for different purposes by the UA:

	<ul>
		<li><a>ink overflow</a>
		<li><a>scrollable 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="https://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.

	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 border boxes
			of all boxes for which it is the containing block,
			accounting for transforms by projecting each box onto
			the plane of the element that establishes its <a>3D rendering context</a>.
			[[!CSS3-TRANSFORMS]]

			Issue: Is this description of handling transforms
			sufficiently accurate?

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

			Note: The 'mask-*' properties [[!CSS-MASKING-1]]
			do not affect the <a>scrollable overflow region</a>.

			Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
	</ul>

	The UA may <em>additionally</em> include
	the margin areas of boxes for which it is the containing block.
	The conditions under which such margin areas are included is undefined in this level.
	<span class="issue">This needs further testing and investigation; is therefore deferred in this draft.</span>
	<!--
	Considerations:
	  * floats
	  * relpos
	  * margins that collapsed through to the edge of the scrollable area
	  * inline-side margins on both blocks and inlines
	Notes:
	  The reasoning for only including border boxes is to minimize the cases
	  in which overflow scrolling is triggered; however once overflow is triggered,
	  excluding margins creates tight, uncomfortable edges
	  because the margin has been effectively truncated.
	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
	-->

	Note: The <a>scrollable overflow rectangle</a> is always a rectangle
	in the box's own coordinate system, but might be non-rectangular
	in other coordinate systems due to transforms [[CSS3-TRANSFORMS]].
	This means scrollbars can sometimes appear when not actually necessary.

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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>scroll container</dfn>
	that allows the user to scroll clipped parts of its <a>scrollable overflow region</a>
	into view.
	The visual viewport of the <a>scroll container</a>
	(through which the <a>scrollable overflow region</a> can be viewed)
	coincides with its padding box,
	and is called the <dfn export>scrollport</dfn>.

	<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>scroll container</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>scroll container</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>scroll container</a>.

		<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>scroll container</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>scroll container</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>scroll containers</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.