Only allow forced fragmentainer breaks at class A break points.

third_party/WebKit/Source/core/layout/LayoutBox.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2006, 2007 Apple Inc. 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
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 611 matching lines @@
     }
 
     // Is the specified break-before or break-after value supported on this object? It needs to be
     // in-flow all the way up to a fragmentation context that supports the specified value.
     bool isBreakBetweenControllable(EBreak) const;
 
     // Is the specified break-inside value supported on this object? It needs to be contained by a
     // fragmentation context that supports the specified value.
     bool isBreakInsideControllable(EBreak) const;
 
-    EBreak breakAfter() const;
-    EBreak breakBefore() const;
+    virtual EBreak breakAfter() const;
+    virtual EBreak breakBefore() const;
     EBreak breakInside() const;
 
+    // Join two adjacent break values specified on break-before and/or break-after. avoid* values
+    // win over auto values, and forced break values win over avoid* values. |firstValue| is
+    // specified on an element earlier in the flow than |secondValue|. This method is used at class
+    // A break points [1], to join the values of the previous break-after and the next
+    // break-before, to figure out whether we may, must, or should not, break at that point. It is
+    // also used when propagating break-before values from first children and break-after values on
+    // last children to their container.
+    //
+    // [1] https://drafts.csswg.org/css-break/#possible-breaks
+    static EBreak joinFragmentainerBreakValues(EBreak firstValue, EBreak secondValue);
+
     static bool isForcedFragmentainerBreakValue(EBreak);
 
-    bool hasForcedBreakBefore() const;
-    bool hasForcedBreakAfter() const;
+    EBreak classABreakPointValue(EBreak previousBreakAfterValue) const;
+
+    // Return true if we should insert a break in front of this box. The box needs to start at a
+    // valid class A break point in order to allow a forced break. To determine whether or not to
+    // break, we also need to know the break-after value of the previous in-flow sibling.
+    bool needsForcedBreakBefore(EBreak previousBreakAfterValue) const;
 
     LayoutRect localOverflowRectForPaintInvalidation() const override;
     bool mapToVisualRectInAncestorSpace(const LayoutBoxModelObject* ancestor, LayoutRect&, VisualRectFlags = DefaultVisualRectFlags) const override;
     virtual void invalidatePaintForOverhangingFloats(bool paintAllDescendants);
 
     LayoutUnit containingBlockLogicalHeightForGetComputedStyle() const;
 
     LayoutUnit containingBlockLogicalWidthForContent() const override;
     LayoutUnit containingBlockLogicalHeightForContent(AvailableLogicalHeightType) const;
 
@@ skipping 490 matching lines @@
         || breakValue == BreakLeft
         || breakValue == BreakPage
         || breakValue == BreakRecto
         || breakValue == BreakRight
         || breakValue == BreakVerso;
 }
 
 } // namespace blink
 
 #endif // LayoutBox_h