Resolve inline margins in over-constrained situations correctly.

third_party/WebKit/Source/core/layout/LayoutBox.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
@@ skipping 2684 matching lines @@
     computed_values.extent_ = ConstrainLogicalWidthByMinMax(
         preferred_width, container_width_in_inline_direction, cb);
   }
 
   // Margin calculations.
   ComputeMarginsForDirection(
       kInlineDirection, cb, container_logical_width, computed_values.extent_,
       computed_values.margins_.start_, computed_values.margins_.end_,
       Style()->MarginStart(), Style()->MarginEnd());
 
+  // CSS 2.1 10.3.3
+  // If 'width' is not 'auto' and 'border-left-width' + 'padding-left' + 'width'
+  //  + 'padding-right' + 'border-right-width' (plus any of 'margin-left' or
+  // 'margin-right' that are not 'auto') is larger than the width of the
+  // containing block, then any 'auto' values for 'margin-left' or
+  // 'margin-right' treated as zero. The following should only apply for parent
+  // width >=  children width
   if (!has_perpendicular_containing_block && container_logical_width &&
-      container_logical_width !=
+      container_logical_width >=

[facetothefate, 2017/04/26 18:09:43]

To keep the old behavior, should be > I guess

           (computed_values.extent_ + computed_values.margins_.start_ +
            computed_values.margins_.end_) &&
       !IsFloating() && !IsInline() && !cb->IsFlexibleBoxIncludingDeprecated() &&
       !cb->IsLayoutGrid()) {
     LayoutUnit new_margin = container_logical_width - computed_values.extent_ -
                             cb->MarginStartForChild(*this);
     bool has_inverted_direction = cb->Style()->IsLeftToRightDirection() !=
                                   Style()->IsLeftToRightDirection();
     if (has_inverted_direction)
       computed_values.margins_.start_ = new_margin;
@@ skipping 3183 matching lines @@
 void LayoutBox::MutableForPainting::
     SavePreviousContentBoxSizeAndLayoutOverflowRect() {
   auto& rare_data = GetLayoutBox().EnsureRareData();
   rare_data.has_previous_content_box_size_and_layout_overflow_rect_ = true;
   rare_data.previous_content_box_size_ = GetLayoutBox().ContentBoxRect().Size();
   rare_data.previous_layout_overflow_rect_ =
       GetLayoutBox().LayoutOverflowRect();
 }
 
 }  // namespace blink