A forced break inside a break-inside:avoid object may make it fit where it is.

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

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2007 David Smith (catfish.man@gmail.com)
  * Copyright (C) 2003-2013 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2010. All rights reserved.
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ skipping 288 matching lines @@
   // page. That would waste space in the container for no good reason, and it
   // would also be a spec violation, since there is no break opportunity defined
   // between the content logical top of an object and its first child or line
   // (only *between* blocks or lines).
   LayoutUnit paginationStrutPropagatedFromChild() const {
     return m_rareData ? m_rareData->m_paginationStrutPropagatedFromChild
                       : LayoutUnit();
   }
   void setPaginationStrutPropagatedFromChild(LayoutUnit);
 
+  LayoutUnit firstForcedBreakOffset() const {
+    if (!m_rareData)
+      return LayoutUnit();
+    return m_rareData->m_firstForcedBreakOffset;
+  }
+  void setFirstForcedBreakOffset(LayoutUnit);
+
   void positionSpannerDescendant(LayoutMultiColumnSpannerPlaceholder& child);
 
   bool avoidsFloats() const override;
 
   using LayoutBoxModelObject::moveChildrenTo;
   void moveChildrenTo(LayoutBoxModelObject* toBoxModelObject,
                       LayoutObject* startChild,
                       LayoutObject* endChild,
                       LayoutObject* beforeChild,
                       bool fullRemoveInsert = false) override;
@@ skipping 353 matching lines @@
     static LayoutUnit positiveMarginAfterDefault(const LayoutBlockFlow* block) {
       return block->marginAfter().clampNegativeToZero();
     }
     static LayoutUnit negativeMarginAfterDefault(const LayoutBlockFlow* block) {
       return (-block->marginAfter()).clampNegativeToZero();
     }
 
     MarginValues m_margins;
     LayoutUnit m_paginationStrutPropagatedFromChild;
 
+    LayoutUnit m_firstForcedBreakOffset;
+
     LayoutMultiColumnFlowThread* m_multiColumnFlowThread;
 
     unsigned m_breakBefore : 4;
     unsigned m_breakAfter : 4;
     int m_lineBreakToAvoidWidow;
     bool m_didBreakAtLineToAvoidWidow : 1;
     bool m_discardMarginBefore : 1;
     bool m_discardMarginAfter : 1;
   };
 
@@ skipping 222 matching lines @@
   void positionDialog();
 
   // END METHODS DEFINED IN LayoutBlockFlowLine
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutBlockFlow, isLayoutBlockFlow());
 
 }  // namespace blink
 
 #endif  // LayoutBlockFlow_h