topics overflow formatting context

Define "formatting context" and fix 'Applies to' line of 'overflow' property

Spec_tags     : css21, css3-flexbox
Owner_tags    : fantasai, antonp, TabAtkins
Status_tags   : Resolved
Added_dt      : 2012-05-15
Action        : React to CfC - http://lists.w3.org/Archives/Public/www-style/2012Jul/0342.html
Issue_urls    : https://www.w3.org/Bugs/Public/show_bug.cgi?id=15381
Proposal_urls : http://lists.w3.org/Archives/Public/www-style/2012May/0494.html
Agenda_urls   :  #If this is part of an ordered series of related topics, e.g. LC issues, use this to link to the supertopic agenda

Problem Statement

The overflow property says it only applies to block containers, but it should also apply to tables and flex containers.

Related Problems

For non-root elements whose position is 'relative' or 'static', the containing block is defined to be formed by the content edge of the nearest block container ancestor box [or table box], but it should be formed by other types of layout container for certain elements in other layout schemes (for example by the flexbox for flexbox items).

Note that the issue of other layout schemes needing such adjustments to overflow and the containing block hierarchy already exists within CSS21 itself:

• Bug 15381 - Incorrect "Applies To" line for 'overflow' property (Error in resolution for Issue 120) - https://www.w3.org/Bugs/Public/show_bug.cgi?id=15381
• Bug 15686 - Containing block for internal tables elements should be table not table wrapper (Error in resolution for Issue 120) - https://www.w3.org/Bugs/Public/show_bug.cgi?id=15686 Both these issues arose through an oversight when introducing the terms "block container" etc, namely that table boxes were overlooked. This illustrates the special-casing that spec editors must remember to perform in individual layout specs if the problem is not solved more generally.

Approach

• Define the concept of a "formatting context". (Alternative possibility: redefine "block formatting context" to not only apply to blocks. See Bug 17121 - Desire to introduce the concept of a "formatting context" for forwards compatibility - https://www.w3.org/Bugs/Public/show_bug.cgi?id=17121 )
• Change the definition of overflow
• Change the containing block hierarchy (optional addition to proposal)

Proposal

Formatting context

In 9.4 (Normal flow), replace:

Boxes in the normal flow belong to a formatting context, which may

be block or inline, but not both simultaneously. Block-level boxes

participate in a block formatting context. Inline-level boxes

participate in an inline formatting context.

with:

| Boxes in the normal flow belong to a formatting context, which in | CSS21 may be block, inline or table. In future levels of CSS, other | types of formatting context will be introduced. Block-level boxes | participate in a block formatting context. Inline-level boxes | participate in an inline formatting context. Table formatting | contexts are described in the chapter on tables.

In 17.4 (Tables in the visual formatting model), replace:

[...] The table wrapper box establishes a block formatting context.

[...]

with:

| [...] The table wrapper box establishes a block formatting context, | and the table box establishes a table formatting context. [...]

In 17.5 (Visual layout of table contents), replace:

Internal table elements generate rectangular boxes with content and

borders. Cells have padding as well. Internal table elements do not

have margins.

with:

| Internal table elements generate rectangular boxes which | participate in the table formatting context established by the | table box. These boxes have content and borders, and cells have | padding as well. Internal table elements do not have margins.

overflow

In 11.1.1 (Overflow), replace:

Applies to: block containers

with:

| Applies to: block containers and boxes that establish a formatting context

Containing block hierarchy (optional addition to proposal)

(This has neither been reviewed nor discussed on the public mailing list.)

In 10.1 (Definition of "containing block"), replace:

2. For other elements, if the element's position is 'relative' or

'static', the containing block is formed by the content edge of

the nearest block container ancestor box.

with:

| 2. For other elements, if the element's position is 'relative' or | 'static', the containing block is formed by the content edge of | the nearest ancestor box that is a block container or which | establishes a formatting context.

Links to More Info

1. Thread start: http://lists.w3.org/Archives/Public/www-style/2012May/0438.html
2. Bug 17122 - "Applies To" line for 'overflow' property hinders forwards-compatibility - https://www.w3.org/Bugs/Public/show_bug.cgi?id=17122
3. Resolution: http://lists.w3.org/Archives/Public/www-style/2012Jun/0475.html , http://lists.w3.org/Archives/Public/www-style/2012Jun/0656.html
4. Call for Censensus on additional clarifying text: http://lists.w3.org/Archives/Public/www-style/2012Jul/0342.html