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[Bug 16037] New: Margin collapsing: unintuitive collapsing between last child and auto-height, large min-height parent

From: <bugzilla@jessica.w3.org>
Date: Mon, 20 Feb 2012 18:46:37 +0000
To: public-css-bugzilla@w3.org
Message-ID: <bug-16037-5148@http.www.w3.org/Bugs/Public/>
https://www.w3.org/Bugs/Public/show_bug.cgi?id=16037

           Summary: Margin collapsing: unintuitive collapsing between last
                    child and auto-height, large min-height parent
           Product: CSS
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: CSS Level 2
        AssignedTo: bert@w3.org
        ReportedBy: antonsforums@yahoo.co.uk
         QAContact: public-css-bugzilla@w3.org


Reported by Anton Prowse

Issue 79 concerned the fact that people didn't like the discontinuity that
arises in earlier versions of the spec when min-height on an auto-height
element transitions from having no effect (in which case the element's bottom
margin collapses with its last in-flow child's bottom margin) to having an
effect (in which case those margins do not collapse): the parent's height is
discontinuously increased to enclose the child's margin.  The WG resolved that
those margins should collapse even when min-height has an effect.
[http://lists.w3.org/Archives/Public/www-style/2008Nov/0473.html]

The motivation behind the resolution was to avoid discontinuities and to avoid
the need to define partial margin collapsing (which was positively viewed in
general but regarded as too complex a change at that point in the spec's life).
 The key test case discussed was when the margin area height of a child with
large bottom margin is slightly bigger than the parent's min-height; it was
felt that the margins should collapse (matching what would have happened if the
parent's min-height had been smaller) rather than have the parent expand its
height to contain the child's margin.

However, this results in strange behaviour: when the parent's min-height is
easily large enough to "contain" the last in-flow child's bottom margin,
collapsing still occurs.  This is unintuitive for authors.

We must decide whether the specified behaviour is still desired.


Issue 79: http://wiki.csswg.org/spec/css2.1#issue-79

URL
    http://lists.w3.org/Archives/Public/www-style/2008Sep/0112.html
Summary
    margin-collapsing with min-height
Proposal
    http://lists.w3.org/Archives/Public/www-style/2008Nov/0297.html
Resolution
    Proposal accepted (telcon 2008-12-10)
Status
    Closed.

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Received on Monday, 20 February 2012 18:46:42 GMT

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