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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/>

           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.

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

    margin-collapsing with min-height
    Proposal accepted (telcon 2008-12-10)

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

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