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Re: [css3-multicol] margin collapsing between column-spanning element and blocks in column boxes: if, when?

From: Håkon Wium Lie <howcome@opera.com>
Date: Tue, 8 Oct 2013 01:11:05 +0200
Message-ID: <21075.16265.657157.955288@gargle.gargle.HOWL>
To: Morten Stenshorne <mstensho@opera.com>
Cc: www-style@gtalbot.org, "W3C www-style mailing list" <www-style@w3.org>
Morten Stenshorne wrote:

 > >  > >  > One issue that I wish would be explicit and clarified in the spec is if
 > >  > >  > margin collapsing can occur between a column-spanning element and blocks
 > >  > >  > in column boxes. If it can occur, then the spec should say so, then define
 > >  > >  > under which conditions along with an example. If it does not occur, then
 > >  > >  > the spec should say so.
 > >  > >
 > >  > > The answer is: yes, margin collapsing occur as per the normal rules.
 > >  > > That is, margin collapsing will not to through the content/border box
 > >  > > of a spanner or a multicol element (both are BFCs), but the margins of
 > >  > > a spanner will collapse with its surroundings.
 > >  > 
 > >  > This has been discussed before, and the opposite conclusion was reached.
 > >  > 
 > >  > http://lists.w3.org/Archives/Public/www-style/2010Oct/0774.html
 > >
 > > You're right that the two conclusions differ, if not being opposite;
 > > it seems to make sense that spanner collapse with others spanners and
 > > its containing block. But not with sibling elements. 
 > 
 > Isn't the containing block of a spanner always the multicol container
 > itself? 

Yes.

 > And since a multicol container establishes a new BFC, its
 > margins cannot collapse with margins of spanner children.

The example I have in mind is a spanner that naturally appears right
before or after a page break. Its bottom (or top) margin will be
"collapsed" with the multicol container. But perhaps "collapsing"
isn't the right term, the spec uses "truncated":

  http://dev.w3.org/csswg/css-break/#break-margins

 > I think a spanner's margins should only collapse with margins of sibling
 > spanners, and nothing else.

Yes, modulo the "truncated" case above.

 > >  > How about this one:
 > >  > 
 > >  >         <div style="columns:2; column-rule:solid;">
 > >  >             <div style="column-span:all; margin-bottom:50px;">spanner</div>
 > >  >             text
 > >  >             <div style="break-before:column; margin-top:50px;">x</div>
 > >  >         </div>
 > >  > 
 > >  > I really don't want those two margins to collapse. :)
 > >
 > > Agree. They wouldn't if we exclud sibling elements. 
 > >
 > > But the second inner DIV (a) could possibly collase with the margins
 > > of the outer DIV if (a) appears after a natural break.
 > 
 > The outer DIV is a multicol container, and multicol containers establish
 > a new BFC, so they cannot collapse with children, can they? 

Correct, this is expressed in example 12.

I've changed example 27 to say:

  Spanners create new block formatting contexts, but their margins can
  be changed by their surroundings. In this example, two spanners
  naturally end up at the top of a page. The top margin of the first
  spanner is truncated due to being after an unforced break. The
  bottom margin of the first spanner collapses with the top margin of
  the second spanner. The bottom margin of the second spanner
  collapses with the top margin of the subsequent element.

to

  Spanners create new block formatting contexts, but their margins can
  be changed by their surroundings. In this example, two spanners
  naturally end up at the top of a page. The top margin of the first
  spanner is truncated due to being after an unforced break. The
  bottom margin of the first spanner collapses with the top margin of
  the second spanner. However, the bottom margin of the second spanner
  does not collabse with the collapses with the top margin of the
  subsequent element.

  http://dev.w3.org/csswg/css-multicol/

Does this seem right?

-h&kon
              Håkon Wium Lie                          CTO °þe®ª
howcome@opera.com                  http://people.opera.com/howcome
Received on Monday, 7 October 2013 23:11:46 UTC

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