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Re: [css-containment][css-regions] how does containement and regions interact

From: Florian Rivoal <florian@rivoal.net>
Date: Wed, 1 Jul 2015 21:30:01 +0200
Cc: www-style list <www-style@w3.org>
Message-Id: <6EF553AF-5107-4D83-96EC-8DAD99619133@rivoal.net>
To: "Tab Atkins Jr." <jackalmage@gmail.com>

> On 01 Jul 2015, at 20:22, Tab Atkins Jr. <jackalmage@gmail.com> wrote:
> On Wed, Jul 1, 2015 at 8:08 AM, Florian Rivoal <florian@rivoal.net> wrote:
>> The current definition of the 'contain' property doesn't work well
>> with regions. flow-from and flow-into are scoped to the contain:style
>> container, but that's not enough, and regions make contain:layout
>> ineffective when used without contain:style.
>> <article>
>>  <div id=container>
>>    <p>Lorem ipsum dolor sit amet...
>>  </div>
>>  <div id=container>
>>    <div>...<table>...<tr>...<td>.../* arbitary nesting */
>>       <p>Aenean vestibulum, dui eu bibendum sagittis...
>>    </td>...</tr>...</table>...</div>
>>  </div>
>>  <p>Sed semper vehicula nisl ac dignissim...
>> </article>
>> <style>
>> container {
>>  contain:layout;
>>  height: 100px;
>>  width: 100px;
>> }
>> p { flow-into: my-flow; }
>> p { flow-from: my-flow; }
>> </style>
>> You can't know what the content of the second or third <p>
>> is going to be (and therefore you cannot lay them out)
>> without first laying out the content of the preceding
>> container. Which also means you cannot figure out the
>> height of <article>, without doing a full layout of all
>> its children, despite the presence of contain:layout on
>> some of them.
>> In the best case, that means that UAs cannot do the type of
>> optimizations you'd expect from contain:layout if regions
>> are involved, and in the worst case, that means UAs can
>> never to the type of optimization you'd expect from
>> contain:layout because regions *might* be involved, and
>> it's too hard to figure out if they actually are.
>> I see 2 ways out of that:
>> 1) merge layout and style containment
>> 2) add a region-specific rule to layout containment in addition
>> to those in style containment. Here's one that should work:
>>  The first (if any) region in a region chain that is either
>>  a contain:layout element itself or the descendant of one is
>>  treated as if it was the last region in the chain, and gets
>>  all the remaining content in the associated named flow.
>>  Subsequent regions in the chain do not receive any content
>>  from the named flow.
>> You'll still need to walk the through the preceding regions
>> in a region chain before you can layout the next one, but
>> if any of them is in a contain:layout element, you don't need
>> to lay it out before you can continue.
>> Actually, we need to reword that so that if several regions
>> of the same chain are descendants of the same contain:layout
>> element, it's the last of one rather than the first, that should
>> behave as the last one in the chain. But that's hard to phase well,
>> so it's probably easier to understand my point with the definition
>> above, even if we need to tweak it before putting it in the spec.
> I'm fine with layout containment also breaking region chains.

By which you mean do what I said, or do the same as contain:style?

 - Florian
Received on Wednesday, 1 July 2015 19:30:31 UTC

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