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RE: [css2.1 issue 79] editorial change for min-height in margin collapsing

From: Alex Mogilevsky <alexmog@microsoft.com>
Date: Mon, 10 Nov 2008 17:49:08 -0800
To: Alex Mogilevsky <alexmog@microsoft.com>, fantasai <fantasai.lists@inkedblade.net>, "www-style@w3.org" <www-style@w3.org>
Message-ID: <7C2F64B551D8664AAD94A28DAC37D0206B4C608AB2@NA-EXMSG-C103.redmond.corp.microsoft.com>
Attached is an example that illustrates that that max-height rule in margin collapsing has the same discontinuity issue that led to removing min-height rule.

-----Original Message-----
From: www-style-request@w3.org [mailto:www-style-request@w3.org] On Behalf Of Alex Mogilevsky
Sent: Friday, November 07, 2008 1:00 PM
To: fantasai; www-style@w3.org
Subject: [css2.1 issue 79] editorial change for min-height in margin collapsing


We have resolved that min-height set on an element doesn't prevent collapsing of its bottom margin with bottom margin of its last child.

I believe it translates to the following editorial change:

        The bottom margin of an in-flow block-level element with a 'height' of 'auto'
        <DEL>and 'min-height' less than the element's used height </DEL>
        and 'max-height' greater than the element's used
        height is adjoining to its last in-flow block-level child's
        bottom margin if the element has no bottom padding or border.

Looking at this change I realize that max-height behavior here has the same kind of discontinuity issue: if used height grows from 1px smaller than max-height to 1px bigger, child margin goes from having effect to having no effect, which makes slightly more sense than min-height case but it is not very intuitive or useful. Should the change include removing max-height from this paragraph too?

        The bottom margin of an in-flow block-level element with a 'height' of 'auto'
        <DEL>and 'min-height' less than the element's used height </DEL>
        <DEL>and 'max-height' greater than the element's used height </DEL>
        is adjoining to its last in-flow block-level child's
        bottom margin if the element has no bottom padding or border.

I think I would prefer the second version.

Also, we have discussed defining "partial collapsing" in cases involving min-height. If we were to define it, it would probably sound like this:

        If last child's bottom margin is positive, for the purpose of
        collapsing with parent's bottom margin it is reduced by the
        difference between min-height and used height, or set to zero
        if the difference is greater than the margin;

        If last child's bottom margin is negative, for the purpose of
        collapsing with parent's bottom margin it is increased by the
        difference between min-height and used height, or set to zero
        if the difference is greater than the absolute value of the margin;

We did not have a consensus however that adding partial collapsing would be an improvement, and none of the browser vendors present have expressed interest in implementing it.

Do we need further discussion/investigation on partial collapsing or can we simply accept one of the two editorial changes above?

Alex

-----Original Message-----
From: www-style-request@w3.org [mailto:www-style-request@w3.org] On Behalf Of fantasai
Sent: Tuesday, November 04, 2008 7:49 PM
To: www-style@w3.org
Subject: [CSSWG] Minutes F2F 2008-10-19

...
Margin collapsing (issue 79)
----------------------------

   RESOLVED: min-height does not turn off margin collapsing



Received on Tuesday, 11 November 2008 01:49:57 GMT

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