Title: CSS Overflow Module Level 3
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/css-overflow-3/
Shortname: css-overflow
Group: csswg
Level: 3
TR: https://www.w3.org/TR/css-overflow-3/
Previous version: https://www.w3.org/TR/2016/WD-css-overflow-3-20160531/
Previous version: https://www.w3.org/TR/2013/WD-css-overflow-3-20130418/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to scrollable overflow handling in visual media.
!Change Log: from 27 January 2015 to the present
!Change Log: from 28 March 2013 to 27 January 2015
!Change Log: from 31 July 2012 to 27 March 2013
Ignored Vars: B, P
Ignored Terms: padding edge, viewport, line box, flex order
At Risk: the 'max-lines' property
Status Text: This update trims away most of the experimental new ideas
for handling overflow that were described in the previous Working Draft.
These ideas are not abandoned; they are merely deferred until Level 4.
Level 3 is focused solely on completing a spec for the existing, shipped 'overflow' features;
work will resume on fragmented overflow and other fun things once this is completed.
spec:css-transforms-2; type:property; text:transform-style
type: dfn; spec:css-multicol-1; text:overflow column
type: dfn; spec:css-transforms-2; text:3d rendering context
url: https://www.w3.org/TR/2008/CR-css3-marquee-20081205/#the-overflow-style; type: property; text: overflow-style;
Introduction
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,
adds a ''overflow/clip'' value,
and defines overflow handling more fully.
[Something something 'max-lines'.]
Types of Overflow
CSS uses the term overflow to describe
the contents of a box
that extend outside one of that box's edges
(i.e., its content edge, padding edge,
border edge, or margin edge).
The term might be interpreted as
elements or features that cause this overflow,
the non-rectangular region occupied by these features,
or, more commonly, as
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 containing block chain
includes the box.
In most cases, overflow
can be computed for any box
from the bounds and properties of that box itself,
plus the overflow
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:
Ink Overflow
The ink overflow 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 scrollable overflow region,
such as box shadows,
border images,
text decoration,
overhanging glyphs (with negative side bearings,
or with ascenders/descenders extending outside the em box),
outlines,
etc.
Since some effects in CSS (for example, the blurs in
'text-shadow' [[CSS-TEXT-3]] and 'box-shadow' [[CSS-BACKGROUNDS-3]],
which are theoretically infinite)
do not define what visual extent they cover, the extent
of the ink overflow is undefined.
The ink overflow region is the non-rectangular area
occupied by the ink overflow
of a box and its contents,
and the ink overflow rectangle is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the ink overflow region.
Note that the ink overflow rectangle is a rectangle
in the box's coordinate system, but might be non-rectangular
in other coordinate systems due to transforms. [[CSS3-TRANSFORMS]]
The scrollable overflow 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 scrollable overflow region is the non-rectangular region
occupied by the scrollable overflow, and the
scrollable overflow rectangle is
the minimal rectangle whose axes are aligned to the box's axes
and that contains the scrollable overflow region.
The scrollable overflow region is the union of:
-
the box’s own content and padding areas
There's disagreement on the scrolling model.
2.1 apparently defined that you scrolled the *content* area;
the content would overflow the content-box,
and you would union that overflow with the content box to find the scrollable area.
In particular, this means that the content would be offset by the start-sides padding,
but if it overflowed,
it would go right to the edge on the end sides.
This is what Firefox and IE do.
At least some authors (and spec authors)
instead have the mental model that the padding box is what's scrollable,
so when you scroll to the end of the overflow,
there's the right/bottom padding.
Chrome/WebKit do this for the block axis, at least.
They're somewhat inconsistent for the inline axis;
there's something weird about how they handle lineboxes.
It seems that the block-axis padding is probably web-compatible to honor.
It's unclear that the inline-axis padding will be.
Further experimentation is needed.
-
all line boxes directly contained by the box
-
the border boxes
of all boxes for which it is the containing block
and whose border boxes are positioned not wholly outside
its block-start or inline-start padding edges,
accounting for transforms by projecting each box onto
the plane of the element that establishes its 3D rendering context.
[[!CSS3-TRANSFORMS]]
Issue: Is this description of handling transforms
sufficiently accurate?
-
the scrollable overflow regions 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 scrollable overflow 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 scrollable overflow region.
Issue: Need to evaluate compat of honoring or ignoring 'clip' and 'clip-path'.
-
Optionally,
additional padding on the end-edge sides,
corresponding to the end-side padding of the scroll container, such that the end edges of its in-flow content
coincide with the end edges of its content area
when scrolled to the end.
Issue(129): It's not yet clear if including the end-side padding in the scrollable layer is Web-compatible, so this clause is under investigation.
It appears that Chrome and Safari include such padding in the block axis;
and the behavior in the inline axis is not clear.
The UA may additionally 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.
This needs further testing and investigation; is therefore deferred in this draft.
Note: The scrollable overflow rectangle 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.
Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow' properties
These properties specify whether a box’s content
(including any ink overflow)
is clipped to its padding edge,
and if so,
whether it is a scroll container
that allows the user to scroll clipped parts of its scrollable overflow region
into view.
The visual viewport of the scroll container
(through which the scrollable overflow region can be viewed)
coincides with its padding box,
and is called the scrollport.
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: as specified, except with ''visible''/''clip'' computing to ''auto''/''hidden'' (respectively) if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''clip''
Animatable: no
The 'overflow-x' property specifies
the handling of overflow 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 overflow in the vertical direction
(i.e., overflow from the top and bottom sides of the box).
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: per grammar
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:
- visible
-
There is no special handling of overflow, that is,
the box’s content is rendered outside the box if positioned there.
The box is not a scroll container.
- hidden
-
This value indicates that
the box’s content is clipped to its padding box
and that the UA must not provide any scrolling user interface
to view the content outside the clipping region,
nor allow scrolling by direct intervention of the user,
such as dragging on a touch screen
or using the scrolling wheel on a mouse.
However, the content must still be scrollable programatically,
for example using the mechanisms defined in [[CSSOM-VIEW]],
and the box is therefore still a scroll container.
- clip
-
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 scroll container.
- scroll
-
This value indicates that
the content is clipped to the padding box,
but can be scrolled into view
(and therefore the box is a scroll container).
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 the target medium is ''print'',
overflowing content may be printed;
it is not defined where it may be printed.
- auto
-
Like ''overflow/scroll'' when the box has scrollable overflow;
like ''overflow/hidden'' otherwise.
Thus, if the user agent uses a scrolling mechanism
that is visible on the screen (such as a scroll bar or a panner),
that mechanism will only be displayed
if there is overflow.
If the computed value of 'overflow' on a block box
is not ''overflow/visible'',
it creates a new block formatting context for its content.
Since scrolling is not possible in static media
(such as print)
authors should be careful to make content accessible in such media,
for example by using ''@media print, (update: none) { … }''
to adjust layout such that all relevant content
is simultaneously visible.
On scroll containers in static media
with an 'overflow' value of ''overflow/auto'' or ''overflow/scroll''
(but not ''overflow/hidden'')
UAs may display an indication of any scrollable overflow,
such as by displaying scrollbars
or an ellipsis.
Note: Not all paged media is static;
for example, e-book readers page content,
but are interactive.
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]].
The initial scroll position,
that is, the initial position of
the box’s scrollable overflow region
with respect to its border box,
prior to any user or programmatic scrolling that changes it,
is dependent on the box’s writing mode,
and is by default the block-start/inline-start edge
of the box’s padding edge.
However, the 'align-content' and 'justify-content' properties [[!CSS-ALIGN-3]]
can be used to change this,
see [[css-align-3#overflow-scroll-position]].
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 scrollable overflow region
of scroll containers
on the block-start and inline-start sides of the box
(thereby behaving as if they had no scrollable overflow on that side).
The viewport uses the principal writing mode for these calculations.
Overflow Viewport Propagation
UAs must apply the 'overflow-*' values
set on the root element to the viewport.
However,
when the root element is an [[!HTML]] <{html}> element
(including XML syntax for HTML)
whose 'overflow' value is ''overflow/visible'' (in both axes),
and that element has a <{body}> element as a child,
user agents must instead apply the 'overflow-*' values
of the first such child element to the viewport.
The element from which the value is propagated must then have
a used 'overflow' value of ''overflow/visible''.
If ''overflow/visible'' is applied to the viewport,
it must be interpreted as ''overflow/auto''.
Limiting Visible Lines: the 'max-lines' property
Name: max-lines
Value: none | <>
Initial: none
Applies to: block containers (excluding multi-column containers)
Inherited: no
Animatable: as integer
Percentages: N/A
Media: visual
Computed value: specified value
This property allows limiting the contents of a block container
to the specified number of lines;
remaining content is fragmented away
and neither rendered nor measured.
Specifically,
if the value of 'max-lines' is not none:
- The box becomes a fragmentation container
that captures region breaks,
if it is not already.
[[!CSS-BREAK-3]]
- A region break
is forced after its Nth
descendant in-flow line box,
where N is the specified value of 'max-lines'.
The contents of descendants that are scroll containers
or that are not block containers
are skipped over while counting line boxes.
If fewer than N line boxes exist,
then 'max-lines' introduces no region break.
- The position of the first forced region break
(whether imposed by 'max-lines' or by another mechanism,
such as the 'break-before'/'break-after' properties)
is treated as the end of the fragmentation container,
and the rest of the fragmented flow is laid out in
the next fragmentation container in the fragmentation context.
If there is no subsequent fragmentation container
(which is the case if the box became a fragmentation container
merely as a consequence of applying the 'max-lines' property),
the rest of the fragmented flow is discarded,
similar to if it were ''display: none'':
it is neither rendered,
nor measured as part of the box’s contents for the purpose of
finding its min-content, max-content, or automatic block sizes,
nor does allow user interaction.
However, since intrinsic sizes are calculated across all fragmentation containers,
this content is taken into account
for the purpose of finding the box’s
min-content and max-content inline sizes
(see [[css-break-3#varying-size-boxes]]).
Note: In the case of parallel fragmentation flows,
content occuring after the fragmentation break in the box tree
could still be rendered,
if it is laid out above the position
representing the end of this fragmentation container.
Only positive integers are accepted.
Zero or negative integers are invalid
and must cause the declaration to be ignored.
Note: The 'widows', 'orphans', and 'break-inside' properties
do not affect the position of the forced region break
introduced by the 'max-lines' property.
Indicating Block-Axis Overflow: the 'block-overflow' property
ISSUE: This section is an unofficial draft, posted here for discussion.
See issue.
Name: block-overflow
Value: clip | ellipsis | <>
Initial: clip
Applies to: block containers
Inherited: yes
Animatable: no
Percentages: N/A
Media: visual
Computed value: specified value
This property allows inserting content into the last line box
before a (forced or unforced) region break
to indicate the continuity of truncated/interrupted content.
It only affects line boxes contained directly by the block container itself,
but as it inherits, will have an effect on descendants’ line boxes unless overridden.
If the box contains no line box immediately preceding a region break,
then this property has no effect.
ISSUE: Should this apply to other types of fragmentation breaks (e.g. pages, columns)?
The inserted content is called the block overflow ellipsis.
Values have the following meanings:
- clip
-
The content is not altered.
- ellipsis
-
Render an ellipsis character (U+2026)--
or a more typographically-appropriate equivalent--
as the block overflow ellipsis
at the end of the affected line box.
UAs should use the conventions of the
content language,
writing system, and
writing mode
to determine the most appropriate ellipsis string.
- <>
-
Render the specified string
as the block overflow ellipsis
at the end of the affected line box.
The UA may truncate this string if it is absurdly long.
In this example, the lead paragraph of each article
is listed in a shortened menu,
truncated to fit within 10 lines
that end with “… (continued on next page)”:
<![CDATA[
li {
max-lines: 5;
block-overflow: "… (continued on next page)";
}
strong {
display: block;
text-transform: uppercase;
}
]]>
<![CDATA[
<li><a href="cheese-is-milk">
<strong>Cheese is Actually Made of Milk!</strong>
Investigative reporters at the World Wide Web Press Corps
have discovered the secret of cheese.
Tracing through byzantine layers of bureacracy and shadow corporations,
our crack team of journalists have traced the source of camembert.
</a></li>
]]>
Sample rendering:
+---------------------------------------+
| CHEESE IS ACTUALLY MADE OF MILK! |
| Investigative reporters at the World |
| Wide Web Press Corps have discovered |
| the secret of cheese. Tracing through |
| byzantine… (continued on next page) |
+---------------------------------------+
Setting 'max-lines' and 'block-overflow' together: the 'line-clamp' property
Name: line-clamp
Value: none | <>
Initial: none
Applies to: see individual properties
Inherited: see individual properties
Animatable: see individual properties
Percentages: N/A
Media: visual
Computed value: see individual properties
The 'line-clamp' property is a shorthand
for the 'max-lines' and 'block-overflow' properties.
- none
- Sets 'max-lines' to ''max-lines/none''
and 'block-overflow' to ''block-overflow/clip''.
- <>
- Sets 'max-lines' to the specified <>
and 'block-overflow' to ''block-overflow/ellipsis''.
For compatibility with legacy content, UAs that support 'line-clamp' must also support -webkit-line-clamp as an alias.
Privacy and Security Considerations {#priv-sec}
===============================================
This specification introduces no new privacy or security concerns.
Acknowledgments
Thanks especially to the feedback from
Rossen Atanassov,
Bert Bos,
Tantek Çelik,
John Daggett,
Daniel Glazman,
Vincent Hardy,
Håkon Wium Lie,
Peter Linss,
Robert O'Callahan,
Florian Rivoal,
Alan Stearns,
Steve Zilles,
and all the rest of the
www-style community.