Title: CSS Overflow Module Level 4
Status: ED
Work Status: Exploring
ED: https://drafts.csswg.org/css-overflow-4/
Shortname: css-overflow
Group: csswg
Level: 4
TR: https://www.w3.org/TR/css-overflow-4/
Previous version: https://www.w3.org/TR/css-overflow-3/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Florian Rivoal, On behalf of Bloomberg, http://florian.rivoal.net/, w3cid 43241
Abstract: This module contains the features of CSS relating to new mechanisms of overflow handling in visual media (e.g., screen or paper).  In interactive media, it describes features that allow the overflow from a fixed size container to be handled by pagination (displaying one page at a time).  It also describes features, applying to all visual media, that allow the contents of an element to be spread across multiple fragments, allowing the contents to flow across multiple regions or to have different styles for different fragments.
Ignored Terms: scroll

type: dfn; spec:css-multicol-1; text:overflow column
spec:css-pseudo-4; type:selector; text:::first-letter
spec:css-pseudo-4; type:selector; text:::first-line
spec:css-writing-modes-4; type:dfn; text:start
spec:css-writing-modes-4; type:dfn; text:end

url: https://drafts.csswg.org/selectors-3/#subject; type: dfn; text: subject;

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 [[CSS2]] 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 was further refined in the CSS Overflow Module Level 3 [[CSS-OVERFLOW-3]].

However, scrolling is not the only way to present large amounts of content, and may even not be the optimal way. After all, the codex replaced the scroll as the common format for large written works because of its advantages.

This specification introduces a mechanism for Web pages to specify that an element of a page should handle overflow through pagination rather than through scrolling.

This specification also extends the concept of overflow in another direction. Instead of requiring that authors specify a single area into which the content of an element must flow, this specification allows authors to specify multiple fragments, each with their own dimensions and styles, so that the content of the element can flow from one to the next, using as many as needed to place the content without overflowing.

In both of these cases, implementations must break the content in the block-progression dimension. Implementations must do this is described in the CSS Fragmentation Module [[!CSS3-BREAK]].

The definition of the 'text-overflow' property in this module supersedes the one in [[CSS-UI-3]] and [[CSS-OVERFLOW-3]].

Value Definitions

Types of overflow

Overflow properties

Overflow Ellipsis: the 'text-overflow' property

Name: text-overflow
Value: [ clip | ellipsis | <> | fade | <> ]{1,2}
Initial: clip
Applies to: block containers
Inherited: no
Percentages: refer to the width of the line box
Computed value: as specified, with lengths made absolute
Animation type: by computed value type

clip

Clip inline content that overflows its block container element. Characters may be only partially rendered.

ellipsis

Render an ellipsis character (U+2026) to represent clipped inline content. Implementations may substitute a more language, script, or writing-mode appropriate ellipsis character, or three dots "..." if the ellipsis character is unavailable.

<>

Render the given string to represent clipped inline content. The given string is treated as an independent paragraph for bidi purposes.

fade( <> | <> )

Clip inline content that overflows its line box. Characters may be only partially rendered. In addition, the UA must apply a fade out effect near the edge of the line box, reaching complete transparency at the edge. Issue: Do we need to define the way the fade out is calculated so that the fading is identical across browsers? It should probably be something like ''mask-image: linear-gradient(to right, rgba(0,0,0,1), rgba(0,0,0,0))'', except applied to the relevant portion of the line only. The argument determines the distance over which the fade effect is applied. The <> is resolved against the width of the line box. Values lower than 0 are clipped to 0. Values greater than the width of the line box are clipped to the width of the line box. Issue: If the line box is too short to display the fade effect at the desired length, should we drop the effect, or shrink the distance it is applied over until it fits, or clip the end of the fade? Issue: How should we deal with things overflowing out of the line box, or overlapping onto it? Should fade apply to the logical content of the line, or to the physical area of the line box, or the intersection of both?

fade

Same as ''fade()'', but the distance over which the fading effect is applied is determined by the UA. ''1em'' is suggested as a reasonable value.

div {
  font-family: monospace;
  white-space: pre;
  overflow: hidden;
  width: 9ch;
  text-overflow: ellipsis;
}

ellipsing details

• Ellipsing only affects rendering and must not affect layout nor dispatching of pointer events: The UA should dispatch any pointer event on the ellipsis to the elided element, as if 'text-overflow' had been ''text-overflow/none''.
• The ellipsis is styled and baseline-aligned according to the block.
• Ellipsing occurs after relative positioning and other graphical transformations.
• If there is insufficient space for the ellipsis, then clip the rendering of the ellipsis itself (on the same side that neutral characters on the line would have otherwise been clipped with the ''text-overflow:clip'' value).

user interaction with ellipsis

• When the user is interacting with content (e.g. editing, selecting, scrolling), the user agent may treat ''text-overflow/ellipsis'', string values, ''overflow-text/fade'' or ''overflow-text/fade()'' as ''text-overflow:clip''.
• Selecting the ellipsis should select the ellipsed text. If all of the ellipsed text is selected, UAs should show selection of the ellipsis. Behavior of partially-selected ellipsed text is up to the UA.

	div {
		font-family: Helvetica, sans-serif;
		line-height: 1.1;
		width: 3.1em;
		border: solid .1em black;
		padding: 0.2em; margin:1em 0;
	}
	

			<div>
			CSS IS AWESOME, YES
			</div>
			

			<div style="text-overflow:clip; overflow:hidden">
			CSS IS AWESOME, YES
			</div>
			

			<div style="text-overflow:ellipsis; overflow:hidden">
			CSS IS AWESOME, YES
			</div>
			

			<div style="text-overflow:ellipsis; overflow:hidden">
			NESTED
			 <p>PARAGRAPH</p>
			WON'T ELLIPSE.
			</div>
			

			<div style="text-overflow:fade; overflow:hidden">
			CSS IS AWESOME, YES
			</div>
			

ellipsis interaction with scrolling interfaces

	div.crawlbar {
		text-overflow: ellipsis;
		height: 2em;
		overflow: scroll;
		white-space: nowrap;
		width: 15em;
		border:1em solid black;
	}
	

	<div class="crawlbar">
	CSS is awesome, especially when you can scroll
	to see extra text instead of just
	having it overlap other text by default.
	</div>
	

Fragmentation of overflow

		Name: continue
		Value: auto | overflow | paginate | fragments | discard
		Initial: auto
		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
		Inherited: no
		Percentages: N/A
		Computed value: see below
		Animation type: discrete
	

auto

''continue/auto'' may only occur as a computed value if the element is a CSS Region other than the last one in a region chain. Content that doesn't fit is pushed to the next region of the chain. In all other cases, ''continue/auto'' computes to one of the other values.

overflow

Content that doesn't fit overflows, according to the 'overflow' property

discard

Content that doesn't fit is discarded at a fragmentation break Note: generalized from region-fragment: break; on the last region of a region chain Issue: When the element isn't a fragmentation container already, should this work by turning it directly into one, or by creating a fragment box inside it like ''continue/fragments'' does?

paginate

Content that doesn't fit paginates. This creates a paginated view inside the element similar to the way that 'overflow: scroll' creates a scrollable view. See paginated overflow Note: Print is effectively "continue: paginate" on the root.

fragments

content that doesn't fit causes the element to copy itself and continue laying out. See fragment overflow.

1. On elements or pseudo elements with layout containment (see [[!CSS-CONTAIN-1]]), if the specified value is ''continue/auto'' or ''continue/fragments'' then the computed value is ''continue/overflow''.
2. Otherwise, if the specified value is ''continue/auto''
   1. On a CSS Region other than the last one in a region chain, the computed value is ''continue/auto''
   2. On a page the computed value is ''continue/paginate''
   3. On a fragment box the computed value is ''continue/fragments''
   4. Otherwise, the computed value is ''continue/overflow''
3. Otherwise, if the specified value is ''continue/fragments''
   1. On a page the computed value is ''continue/paginate''
   2. Otherwise, the computed value is the specified value
4. In all other cases, the computed value is the specified value

Paginated overflow

@media (overflow-block: paged), (overflow-block: optional-paged) {
	:root {
		continue: paginate;
	}
}

The current implementation of paginated overflow uses the overflow/overflow-x/overflow-y properties rather than the overflow-style property as proposed in the [[CSS3GCPM]] draft (which also matches the [[CSS3-MARQUEE]] proposal). or the 'continue' property as described here.

Fragment overflow

When the computed value of 'continue' for an element is ''continue/fragments'', and implementations would otherwise have created a box for the element, then implementations must create a sequence of fragment boxes for that element. (It is possible for an element with ''continue: fragments'' to generate only one fragment box. However, if an element's computed 'continue' is not ''continue/fragments'', then its box is not a fragment box.) Every fragment box is a fragmentation container, and any overflow that would cause that fragmentation container to fragment causes another fragment box created as a next sibling of the previous one. Or is it as though it's a next sibling of the element? Need to figure out exactly how this interacts with other box-level fixup. Additionally, if the fragment box is also a multi-column box (as defined in [[!CSS3COL]] though it defines multi-column container) any content that would lead to the creation of overflow columns [[!CSS3COL]] instead is flown into an additional fragment box. However, fragment boxes may themselves be broken (due to fragmentation in a fragmentation context outside of them, such as pages, columns, or other fragment boxes); such breaking leads to fragments of the same fragment box rather than multiple fragment boxes. (This matters because fragment boxes may be styled by their index; such breaking leads to multiple fragments of a fragment box with a single index. This design choice is so that breaking a fragment box across pages does not break the association of indices to particular pieces of content.) Should a forced break that breaks to an outer fragmentation context cause a new fragment of a single fragment box or a new fragment box? Should we find a term other than fragment box here to make this a little less confusing?

What if we want to be able to style the pieces of an element split within another type of fragmentation context? These rules prevent ever using ''::nth-fragment()'' for that, despite that the name seems the most logical name for such a feature.

<!DOCTYPE HTML>
<title>Breaking content into
  equal-sized cards</title>
<style>
  .in-cards {
    continue: fragments;

    width: 13em;
    height: 8em;

    padding: 4px;
    border: medium solid blue;
    margin: 6px;

    font: medium/1.3 Times New
      Roman, Times, serif;
  }
</style>
<div class="in-cards">
  In this example, the text in the div
  is broken into a series of cards.
  These cards all have the same style.
  The presence of enough content to
  overflow one of the cards causes
  another one to be created.  The second
  card is created just like it's the
  next sibling of the first.
</div>

We should specify that ''continue: fragments'' does not apply to at least some table parts, and perhaps other elements as well. We need to determine exactly which ones.

This specification needs to say which type of fragmentation context is created so that it's clear which values of the 'break-*' properties cause breaks within this context. We probably want ''break-*: region'' to apply.

This specification needs a processing model that will apply in cases where the layout containing the fragments has characteristics that use the intrinsic size of the fragments to change the amount of space available for them, such as [[CSS3-GRID-LAYOUT]]. There has already been some work on such a processing model in [[CSS3-REGIONS]], and the work done on a model there, and the editors of that specification, should inform what happens in this specification.

Fragment styling

The ''::nth-fragment()'' pseudo-element

The ::nth-fragment() pseudo-element is a pseudo-element that describes some of the fragment boxes generated by an element. The argument to the pseudo-element takes the same syntax as the argument to the :nth-child() pseudo-class defined in [[!SELECT]], and has the same meaning except that the number is relative to fragment boxes generated by the element instead of siblings of the element.

Selectors that allow addressing fragments by counting from the end rather than the start are intentionally not provided. Such selectors would interfere with determining the number of fragments.

Depending on future discussions, this ''::nth-fragment(an+b)'' syntax may be replaced with the new ''::fragment:nth(an+b)'' syntax.

Styling of fragments

Should this apply to continue:fragments only, or also to continue:paginate? (If it applies, then stricter property restrictions would be needed for continue:paginate.)

In the absence of rules with ''::nth-fragment()'' pseudo-elements, the computed style for each fragment box is the computed style for the element for which the fragment box was created. However, the style for a fragment box is also influenced by rules whose selector's subject [[!SELECT]] has an ''::nth-fragment()'' pseudo-element, if the 1-based number of the fragment box matches that ''::nth-fragment()'' pseudo-element and the selector (excluding the ''::nth-fragment()'' pseudo-element) matches the element generating the fragments.

When determining the style of the fragment box, these rules that match the fragment pseudo-element cascade together with the rules that match the element, with the fragment pseudo-element adding the specificity of a pseudo-class to the specificity calculation. Does this need to be specified in the cascading module as well?

<!DOCTYPE HTML>
<style>
  .bouncy-columns {
    continue: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .bouncy-columns::nth-fragment(1) {
    background: aqua; color: black;
    transform: rotate(-3deg);
  }
  .bouncy-columns::nth-fragment(2) {
    background: yellow; color: black;
    transform: rotate(3deg);
  }
</style>
<div class="bouncy-columns">
  ...
</div>

Styling an ''::nth-fragment()'' pseudo-element with the 'continue' property does take effect; if a fragment box has a computed value of 'continue' other than ''fragments'' then that fragment box is the last fragment. However, overriding 'continue' on the first fragment does not cause the fragment box not to exist; whether there are fragment boxes at all is determined by the computed value of overflow for the element.

Styling an ''::nth-fragment()'' pseudo-element with the 'content' property has no effect; the computed value of 'content' for the fragment box remains the same as the computed value of content for the element.

Specifying ''display: none'' for a fragment box causes the fragment box with that index not to be generated. However, in terms of the indices used for matching ''::nth-fragment()'' pseudo-elements of later fragment boxes, it still counts as though it was generated. However, since it is not generated, it does not contain any content.

Specifying other values of 'display', 'position', or 'float' is permitted, but is not allowed to change the inner display type. (Since 'continue' only applies to block containers, flex containers, and grid containers). Need to specify exactly how this works

To match the model for other pseudo-elements where the pseudo-elements live inside their corresponding element, declarations in ''::nth-fragment()'' pseudo-elements override declarations in rules without the pseudo-element. The relative priority within such declarations is determined by normal cascading order (see [[!CSS2]]).

Styles specified on ''::nth-fragment()'' pseudo-elements do affect inheritance to content within the fragment box. In other words, the content within the fragment box must inherit from the fragment box's style (i.e., the pseudo-element style) rather than directly from the element. This means that elements split between fragment boxes may have different styles for different parts of the element.

This inheritance rule allows specifying styles indirectly (by using explicit ''inherit'' or using default inheritance on properties that don't apply to ''::first-letter'') that can't be specified directly (based on the rules in the next section). This is a problem. The restrictions that apply to styling inside fragments should also apply to inheritance from fragments.

<!DOCTYPE HTML>
<style>
  .article {
    continue: fragments;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    margin-bottom: 1em;
    height: 4em;
  }
  .article::nth-fragment(2) {
    margin-left: 5em;
    margin-right: 2em;
  }
</style>
<div class="article">
  The <code>font-size</code> property...
</div>

Styling inside fragments

Should this apply to continue:fragments only, or also to continue:paginate?

The ''::nth-fragment()'' pseudo-element can also be used to style content inside of a fragment box. Unlike the ''::first-line'' and ''::first-letter'' pseudo-elements, the ''::nth-fragment()'' pseudo-element can be applied to parts of the selector other than the subject: in particular, it can match ancestors of the subject. However, the only CSS properties applied by rules with such selectors are those that apply to the ''::first-letter'' pseudo-element.

To be more precise, when a rule's selector has ''::nth-fragment()'' pseudo-elements attached to parts of the selector other than the subject, the declarations in that rule apply to a fragment (or pseudo-element thereof) when:

1. the declarations are for properties that apply to the ''::first-letter'' pseudo-element,
2. the declarations would apply to that fragment (or pseudo-element thereof) had those ''::nth-fragment()'' pseudo-elements been removed, with a particular association between each sequence of simple selectors and the element it matched, and
3. for each removed ''::nth-fragment()'' pseudo-element, the fragment lives within a fragment box of the element associated in that association with the selector that the pseudo-element was attached to, and whose index matches the pseudo-element.

<!DOCTYPE HTML>
<style>
  .dark-columns {
    continue: fragments;
    width: 6em;
    height: 10em;
    float: left;
    margin-right: 1em;
    font: medium/1.25 Times New
      Roman, Times, serif;
  }
  .dark-columns::nth-fragment(1) {
    background: aqua; color: black;
  }
  .dark-columns::nth-fragment(1) :link {
    color: blue;
  }
  .dark-columns::nth-fragment(1) :visited {
    color: purple;
  }
  .dark-columns::nth-fragment(2) {
    background: navy; color: white;
  }
  .dark-columns::nth-fragment(2) :link {
    color: aqua;
  }
  .dark-columns::nth-fragment(2) :visited {
    color: fuchsia;
  }
</style>
<div class="dark-columns">
  ...
</div>

The 'max-lines' property

Authors may wish to style the opening lines of an element with different styles by putting those opening lines in a separate fragment. However, since it may be difficult to predict the exact height occupied by those lines in order to restrict the first fragment to that height, this specification introduces a 'max-lines' property that forces a fragment to break after a specified number of lines. This forces a break after the given number of lines contained within the element or its descendants, as long as those lines are in the same block formatting context.

		Name: max-lines
		Value: none | <>
		Initial: none
		Applies to: fragment boxes
		Inherited: no
		Animatable: as integer
		Percentages: N/A
		Computed value: the keyword ''max-lines/none'' or an integer
		Animation type: by computed value type
	

none

Breaks occur only as specified elsewhere.

<>

In addition to any breaks specified elsewhere, a break is forced before any line that would exceed the given number of lines being placed inside the element (excluding lines that are in a different block formatting context from the block formatting context to which an unstyled child of the element would belong).

If there are multiple boundaries between this line and the previous, where exactly (in terms of element boundaries) is the break forced?

Only positive integers are accepted. Zero or negative integers are a parse error.

Should this apply to fragment overflow only, or also to pagination? Given what we're doing with the continue property, it should actually apply to any fragmentainer.

<!DOCTYPE HTML>
<style>
  .article {
    continue: fragments;
  }
  .article::first-letter {
    font-size: 2em;
    line-height: 0.9;
  }
  .article::nth-fragment(1) {
    font-size: 1.5em;
    max-lines: 3;
  }
  .article::nth-fragment(2) {
    column-count: 2;
  }
</style>
<div class="article">
  ...
</div>

Appendix A: Possible extensions for ''scrollbar-gutter''

Issue: This section documents current attempts at extending the 'scrollbar-gutter' property to solve additional use cases. However, it does not currently have consensus. It is presented here to encourage discussion, but non-experimental implementation is not recommended.

This example exercises all the additional values of the 'scrollbar-gutter' property: * ''scrollbar-gutter: always force'' on the header/toolbar * ''scrollbar-gutter: always'' on the scroll container * ''scrollbar-gutter: match-parent'' on each row inside the scroll container

Name: scrollbar-gutter
New Values: auto | [ [ stable | always ] && both-edges? && force? ] || match-parent
Applies to: all elements

For overlay scrollbars, the precise width of the scrollbar gutter, if present, is UA defined. However, it must not be 0, and it must not change based on user interactions with the page or the scrollbar even if the scrollbar itself changes, with the expectation that it covers the width of the overlay scrollbar in its widest form, to the extent that this is well defined. The new values of this property have the following meaning:

always

The scrollbar gutter is always present when 'overflow' is ''overflow/scroll'', ''overflow/hidden'', or ''overflow/auto'', regardless of the type of scrollbar or of whether the box is overflowing.

''scrollbar-gutter: always'' can be bused to solve the problem of (small) interactive elements near the edge of the element getting covered by an appearing overlay scrollbar. A representative case would be a basic todo list, with each line starting with some text and ending with a right-aligned checkbox. With a classic scrollbar, everything is fine, but an overlay scrollbar could obscure the check boxes and make them hard to interact with.

Overlay Scrollbar are typically transient and disappear when not interacted with, so the checkboxes they cover are not impossible to use. But when the scrollbar is shown it does get in the way, and that makes for an awkward interaction. The author might try and solve the problem by adding some right padding, but (1) how much?, and (2) that padding isn't needed in the case of classic scrollbars. ''scrollbar-gutter: always'' solves this problem, yielding an identical result in first case of classic scrollbars, but adding the desired gutter with overlay scrollbars:

Apple is reluctant to add this value, as authors may use it too broadly, inserting gutters with overlay scrollbars even when not justified by interactive elements, defeating the space-saving advantage of overlay scrollbars. An alternative solution has been suggested: as the focus is interactive elements, maybe we could have a property that applies to the elements that needs to avoid being under the scrollbar. When turned on, it would enlarge the right or left margin of the element as appropriate by just the right value to push it out from under an overlay scrollbar if that's where it would end up, but would leave the element unchanged otherwise. Possibly, an addition toggle would cause the element to enlarge both its inline-end and inline-start margins or neither, rather than just one. This could typically be useful for block-level descendants of the scroller with visible borders or background: adding space on one side to avoid collisions with the overlay scrollbar would make them look off-center when the scrollbar disapears. Increasing the margin on both sides avoids that. Yet another possibility is to have a choice between growing the margin to protect the element, or growing the padding to protect the element's content. The syntax could be something like scrollbar-avoid: none | [self | content] && both-edges?. An interesting consideration is that this may alleviate the need for ''scrollbar-gutter: match-parent'', as it seems that situations that would have been addressed by ''scrollbar-gutter: stable'' or ''scrollbar-gutter: always'' on the parent and ''scrollbar-gutter: match-parent'' on the select children could instead be addressed by leaving the parent as ''scrollbar-gutter: auto'' and using scrollbar-avoid: self or scrollbar-avoid: content on the relevant children.

force

When the ''scrollbar-gutter/force'' keyword is present ''scrollbar-gutter/stable'' and ''scrollbar-gutter/always'' take effect when 'overflow' is ''overflow/visible'', ''overflow/hidden'' or ''overflow/clip'' in addition ''overflow/auto'' or ''overflow/scroll''. This does not cause a scrollbar to be displayed, only a scrollbar gutter.

This value enables authors to reserve the same amount of space on the edges of an element that is adjacent to a scroller as is being reserved in the scroller itself, so that their content would visually line up. The same effect could be achieved by adding padding for classic scrollbars and none for overlay scrollbars, but without this there is no reliable way for authors to know if they should be adding that padding, and if so how much. A concrete example can be seen (at the time of writing) in the UI of Gmail: it attempts, not always successfully, to line up the controls in the toolbar above the list of mails with those in the list of mails. Here are two screenshots, one with classic scrollbars turned on (at the OS level), where Gmail correctly guesses how much padding it should be adding to the toolbar above the list, and one with overlay scrollbars turned on, where Gmail incorrectly adds padding, throwing things out of alignment (orange dotted line added manually to highlight the point under discussion).

Creating this spacing in the toolbar using ''scrollbar-gutter: stable force'' rather than padding would keep icons in alignment in both cases, as well as on systems where the scrollbar has an atypical size.

Issue: It is because of this value that the property has been be made to apply to all elements, rather than merely to [=scroll containers=], so that it can apply to ''overflow: visible'' elements as shown in the example. This could pause implementations difficulties, as user agents cannot just rely on existing code to place the gutter, since they may have to do so on elements which previously could not have one. Restricting it to all elements where 'overflow' applies would probably have no negative effect on use cases and could make implementation easier; however, even that may still be more difficult that limiting it to [=scroll containers=]. Issue: In addition to the implementation challenges mentioned above, it is not clear that this value solves the problem as reliably as it intends to. As the size and side of the scrollbar and its gutter are UA defined, they may vary between different elements. Since the appearance and position of scrollbars are up to the user agent, there is no limit to the list of properties that might influence them. It is probable that setting properties such as 'direction' (via setting the HTML dir attribute) or 'scrollbar-width' could give enough context to the user agent to know how it would create a scrollbar on that element if it were scrollable, and therefore to create a gutter of the same size at the right place, but it cannot be guaranteed. Issue: In early iterations of the specification, this value was the only way to achieve the effect described in the example. However, since then, ''scrollbar-gutter: stable'' has been made to apply to ''overflow: hidden'' elements. While applying ''overflow: hidden'' has other effects which may be undesirable in that context, the combination of ''scrollbar-gutter: stable'' and ''overflow: hidden'' does add spacing in the same way as ''scrollbar-gutter: stable force'', and may prove to be a sufficient workaround, particularly considering the other issues described above.

match-parent

On a [=block-level=] box whose parent has a [=scrollbar gutter=] (or gutters on both edges), this causes the box to have [=scrollbar gutter=] on the same side(s) and of the same width as its parent’s gutter(s). Moreover, that gutter is made to overlap that of the parent box.

If the box with ''scrollbar-gutter: match-parent'' has a non-zero border or margin on the side where the gutter is expected, then the size of that box's gutter is parent.gutter - child.border - child.margin, and the gutter+border+margin is what collapses with the parent's gutter. If the box with ''scrollbar-gutter/ match-parent'' is itself a [=scroll container=], depending on the type of scrollbars, on its 'overflow' property, and on the other values of the 'scrollbar-gutter' property, it may need additional gutter for its own scrollbars. This comes in addition to the amount of gutter added for the sake of the ''scrollbar-gutter/match-parent'' value and does not collapse with the parent's gutter.

Note: The following table summarizes the interaction of 'overflow' and 'scrollbar-gutter' for different types of scrollbars, showing in which case space is reserved for the scrollbar gutter.

Should space be reserved for the [=scrollbar gutter=]?

'overflow'	'scrollbar-gutter'	Classic scrollbars		Overlay scrollbars (whether overflowing or not)
		Overflowing	Not overflowing
''overflow/scroll''	''scrollbar-gutter/auto''	yes	yes
	''scrollbar-gutter/stable''	yes	yes
	''scrollbar-gutter/always''	yes	yes	yes
''overflow/auto''	''scrollbar-gutter/auto''	yes
	''scrollbar-gutter/stable''	yes	yes
	''scrollbar-gutter/always''	yes	yes	yes
''overflow/hidden''	''scrollbar-gutter/auto''
	''scrollbar-gutter/stable''	yes	yes
	''scrollbar-gutter/always''	yes	yes	yes
''overflow/visible'', ''overflow/clip''	''scrollbar-gutter/auto''
	''scrollbar-gutter/stable''	if ''force''	if ''force''
	''scrollbar-gutter/always''	if ''force''	if ''force''	if ''force''

Appendix B: Privacy and Security Considerations

Does this specification deal with personally-identifiable information?

No.

Does this specification deal with high-value data?

No.

Does this specification introduce new state for an origin that persists across browsing sessions?

No.

Does this specification expose persistent, cross-origin state to the web?

No.

Does this specification expose any other data to an origin that it doesn’t currently have access to?

No.

Does this specification enable new script execution/loading mechanisms?

No.

Does this specification allow an origin access to a user’s location?

No.

Does this specification allow an origin access to sensors on a user’s device?

No.

Does this specification allow an origin access to aspects of a user’s local computing environment?

No.

Does this specification allow an origin access to other devices?

No.

Does this specification allow an origin some measure of control over a user agent’s native UI?

No

Does this specification expose temporary identifiers to the web?

No.

Does this specification distinguish between behavior in first-party and third-party contexts?

No.

How should this specification work in the context of a user agent’s "incognito" mode?

No difference in behavior is needed.

Does this specification persist data to a user’s local device?

No.

Does this specification have a "Security Considerations" and "Privacy Considerations" section?

Yes, this is the section you are currently reading.

Does this specification allow downgrading default security characteristics?

No.

Changes

Changes Since Level 3

• The Fragmentation of overflow, Paginated overflow, and Fragment overflow sections and subsections have been removed form Level 3 and moved to Level 4.

Acknowledgments

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 www-style community.