[Webkit-unassigned] [Bug 176620] New: In regular block layout, the width of a child's margin box should always be equal to that of its containing block

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=176620

            Bug ID: 176620
           Summary: In regular block layout, the width of a child's margin
                    box should always be equal to that of its containing
                    block
           Product: WebKit
           Version: Safari 10
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: facetothefate at gmail.com
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

Hi,

Orignally, I have filed a bug in blink here:
https://bugs.chromium.org/p/chromium/issues/detail?id=708751

Today I find I repoduce it on my safari.


After checking the code, it looks we have same root cause here

        LayoutUnit newMargin = containerLogicalWidth - computedValues.m_extent - cb.marginStartForChild(*this);
        bool hasInvertedDirection = cb.style().isLeftToRightDirection() != style().isLeftToRightDirection();
        if (hasInvertedDirection)
            computedValues.m_margins.m_start = newMargin;
        else
            computedValues.m_margins.m_end = newMargin;


Simply because at this time marginStartForChild have not been set yet.

if you saw here:

void RenderBox::updateLogicalWidth()
{
    LogicalExtentComputedValues computedValues;
    computeLogicalWidthInRegion(computedValues);

    setLogicalWidth(computedValues.m_extent);
    setLogicalLeft(computedValues.m_position);
    setMarginStart(computedValues.m_margins.m_start);
    setMarginEnd(computedValues.m_margins.m_end);
}

we are calling computeLogicalWidthInRegion first, then use the result to update the margin.

Therefore below path will fix it as I did for blink:

        LayoutUnit newMarginTotal = containerLogicalWidth - computedValues.m_extent;
        bool hasInvertedDirection = cb.style().isLeftToRightDirection() != style().isLeftToRightDirection();
        if (hasInvertedDirection)
            computedValues.m_margins.m_start = 
                 newMarginTotal - computedValues..m_margins.m_end;
        else
            computedValues.m_margins.m_end = 
                 newMarginTotal - computedValues..m_margins.m_start;

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