W3C home > Mailing lists > Public > www-style@w3.org > March 2013

Re: [css3-multicol] 'column-fill' and column overflow

From: David Hyatt <hyatt@apple.com>
Date: Sun, 03 Mar 2013 12:06:26 -0600
Cc: www-style@w3.org
Message-id: <1579F4BC-9F59-432D-BA19-20F32B4F5742@apple.com>
To: "L. David Baron" <dbaron@dbaron.org>
On Mar 2, 2013, at 3:38 PM, L. David Baron <dbaron@dbaron.org> wrote:

> Second, it seems odd that the distinction it draws is between being
> *in* the overflow columns rather than whether the container *has*
> overflow columns.  By this, consider the example of a multi-column
> container with 2 non-overflow columns and a large unbreakable piece
> of content at the start of the third column.  It seems odd to lay
> this out as:
> 
>  +---------------------------+
>  |+-----------+ +-----------+|+-----------+ +-----------+
>  ||text text  | |text text  |||+--------+ | |text text  |
>  ||text text  | |text text  ||||IMAGE   | | |           |
>  ||text text  | |text text  ||||        | | |           |
>  ||text text  | |text text  |||+--------+ | |           |
>  ||           | |           |||text text  | |           |
>  |+-----------+ +-----------+|+-----------+ +-----------+
>  +---------------------------+
> 
> with balancing happening between the first two columns, but not
> happening in the overflow columns.  It makes more sense to me to
> disable the balancing in continuous context when overflow columns
> are *present*, and only use balancing in continuous contexts when
> there is excess height within the multicolumn element and no
> overflow columns.  That seems to me to be the main use case for
> balancing; having to do balancing in the other cases seems to me
> like extra implementation complexity and reduced performance without
> strong justification.
> 

I think the spec needs to really spell out when and how balancing is supposed to occur, because right now I really have no idea when or how I'm supposed to balance columns. The current implementation of balancing in WebKit is pretty poor as a result, since the spec hasn't really provided enough information for me to really understand what I'm supposed to do. I have no idea how forced breaks affect balancing. I have no idea how overflow affects balancing. There is no balancing algorithm in the spec, so I don't know what the expectations are for balancing (is it 2-pass, is it n-pass, etc.).

My own intuition regarding balancing is that it is done per-column-row, so it seems like the spec should be talking in terms of column-rows. I would think that balancing would be done when:

(1) The height of the column-row is both unknown and not constrained by any enclosing pagination context, so you have to create equal height columns to try to determine the height of the column-row. This could include overflow columns, the column-row on the last printed page, etc.  I don't think you would want to special case overflow in this situation, since you would not even know you had overflow columns until you had done a layout with the balancing guess.

(2) There is excess height in the column-row. Even then I have questions… if the excess height is caused by a forced page break for example, but all the columns in the row had unforced breaks, do you balance the columns on that page?

I really think we need clear and precise rules for exactly when to balance, and I think the spec needs to actually provide an algorithm for balancing, even if it's just informative. Something on the level of

http://dev.w3.org/csswg/css3-regions/#regions-visual-formatting-examples

but explaining how to balance would be helpful.

dave
(hyatt@apple.com)
Received on Sunday, 3 March 2013 18:06:47 GMT

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