Margins should expand/shrink an element's width if no float constrains it

Source/core/rendering/RenderBox.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2005 Allan Sandfeld Jensen (kde@carewolf.com)
  *           (C) 2005, 2006 Samuel Weinig (sam.weinig@gmail.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
@@ skipping 1666 matching lines @@
         return 0;
     LayoutUnit contentSideWithMargin = contentSide + childMargin;
     if (offset > contentSideWithMargin)
         return childMargin;
     return offset - contentSide;
 }
 
 LayoutUnit RenderBox::shrinkLogicalWidthToAvoidFloats(LayoutUnit childMarginStart, LayoutUnit childMarginEnd, const RenderBlockFlow* cb) const
 {
     LayoutUnit logicalTopPosition = logicalTop();
+    LayoutUnit startOffsetForContent = cb->startOffsetForContent();
+    LayoutUnit endOffsetForContent = cb->endOffsetForContent();
+    LayoutUnit startOffsetForLine = cb->startOffsetForLine(logicalTopPosition, false);
+    LayoutUnit endOffsetForLine = cb->endOffsetForLine(logicalTopPosition, false);
+
+    // If there aren't any floats constraining us then allow the margins to shrink/expand the width as much as they want.
+    if (startOffsetForContent == startOffsetForLine && endOffsetForContent == endOffsetForLine)
+        return cb->availableLogicalWidthForLine(logicalTopPosition, false) - childMarginStart - childMarginEnd;
+
     LayoutUnit width = cb->availableLogicalWidthForLine(logicalTopPosition, false) - std::max<LayoutUnit>(0, childMarginStart) - std::max<LayoutUnit>(0, childMarginEnd);
-
     // We need to see if margins on either the start side or the end side can contain the floats in question. If they can,
     // then just using the line width is inaccurate. In the case where a float completely fits, we don't need to use the line
     // offset at all, but can instead push all the way to the content edge of the containing block. In the case where the float
     // doesn't fit, we can use the line offset, but we need to grow it by the margin to reflect the fact that the margin was
     // "consumed" by the float. Negative margins aren't consumed by the float, and so we ignore them.
-    width += portionOfMarginNotConsumedByFloat(childMarginStart, cb->startOffsetForContent(), cb->startOffsetForLine(logicalTopPosition, false));
-    width += portionOfMarginNotConsumedByFloat(childMarginEnd, cb->endOffsetForContent(), cb->endOffsetForLine(logicalTopPosition, false));
+    width += portionOfMarginNotConsumedByFloat(childMarginStart, startOffsetForContent, startOffsetForLine);
+    width += portionOfMarginNotConsumedByFloat(childMarginEnd, endOffsetForContent, endOffsetForLine);
     return width;
 }
 
 LayoutUnit RenderBox::containingBlockLogicalWidthForContent() const
 {
     if (hasOverrideContainingBlockLogicalWidth())
         return overrideContainingBlockContentLogicalWidth();
 
     RenderBlock* cb = containingBlock();
     return cb->availableLogicalWidth();
@@ skipping 3040 matching lines @@
 RenderBox::BoxDecorationData::BoxDecorationData(const RenderStyle& style)
 {
     backgroundColor = style.visitedDependentColor(CSSPropertyBackgroundColor);
     hasBackground = backgroundColor.alpha() || style.hasBackgroundImage();
     ASSERT(hasBackground == style.hasBackground());
     hasBorder = style.hasBorder();
     hasAppearance = style.hasAppearance();
 }
 
 } // namespace blink