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

From: Florian Rivoal <florian@rivoal.net>
Date: Wed, 1 Jul 2015 17:08:38 +0200
Message-Id: <EF27E698-1B0A-47DB-ACF1-DACE59126A03@rivoal.net>
To: www-style list <www-style@w3.org>
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.

 - Florian
Received on Wednesday, 1 July 2015 15:09:28 UTC

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