Keep block pagination struts after layout, and store them before any type of break.

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 58 matching lines @@
 
     LayoutUnit m_overrideLogicalContentHeight;
     LayoutUnit m_overrideLogicalContentWidth;
 
     // Set by LayoutBox::savePreviousBoxSizesIfNeeded().
     LayoutSize m_previousBorderBoxSize;
     LayoutRect m_previousContentBoxRect;
     LayoutRect m_previousLayoutOverflowRect;
 
     LayoutUnit m_pageLogicalOffset;
+
+    LayoutUnit m_paginationStrut;
 };
 
 class CORE_EXPORT LayoutBox : public LayoutBoxModelObject {
 public:
     explicit LayoutBox(ContainerNode*);
 
     PaintLayerType layerTypeRequired() const override;
 
     bool backgroundIsKnownToBeOpaqueInRect(const LayoutRect& localRect) const override;
 
@@ skipping 387 matching lines @@
     // the replaced LayoutObject to quickly determine what line it is contained on and to easily
     // iterate over structures on the line.
     InlineBox* inlineBoxWrapper() const { return m_rareData ? m_rareData->m_inlineBoxWrapper : 0; }
     void setInlineBoxWrapper(InlineBox*);
     void deleteLineBoxWrapper();
 
     void setSpannerPlaceholder(LayoutMultiColumnSpannerPlaceholder&);
     void clearSpannerPlaceholder();
     LayoutMultiColumnSpannerPlaceholder* spannerPlaceholder() const final { return m_rareData ? m_rareData->m_spannerPlaceholder : 0; }
 
+    // A pagination strut is the amount of space needed to push an in-flow block-level object (or
+    // float) to the logical top of the next page or column. It will be set both for forced breaks
+    // (e.g. page-break-before:always) and soft breaks (when there's not enough space in the current
+    // page / column for the object). The strut is baked into the logicalTop() of the object, so
+    // that logicalTop() - paginationStrut() == the original position in the previous column before
+    // deciding to break.
+    //
+    // Pagination struts are either set in front of a block-level box (here) or before a line
+    // (RootInlineBox::paginationStrut()).
+    LayoutUnit paginationStrut() const { return m_rareData ? m_rareData->m_paginationStrut : LayoutUnit(); }
+    void setPaginationStrut(LayoutUnit);
+    void resetPaginationStrut()
+    {
+        if (m_rareData)
+            m_rareData->m_paginationStrut = LayoutUnit();
+    }
+
     LayoutRect clippedOverflowRectForPaintInvalidation(const LayoutBoxModelObject* paintInvalidationContainer, const PaintInvalidationState* = nullptr) const override;
     void mapRectToPaintInvalidationBacking(const LayoutBoxModelObject* paintInvalidationContainer, LayoutRect&, const PaintInvalidationState*) const override;
     virtual void invalidatePaintForOverhangingFloats(bool paintAllDescendants);
 
     LayoutUnit containingBlockLogicalHeightForGetComputedStyle() const;
 
     LayoutUnit containingBlockLogicalWidthForContent() const override;
     LayoutUnit containingBlockLogicalHeightForContent(AvailableLogicalHeightType) const;
 
     LayoutUnit containingBlockAvailableLineWidth() const;
@@ skipping 441 matching lines @@
         if (UNLIKELY(inlineBoxWrapper() != nullptr))
             deleteLineBoxWrapper();
     }
 
     ensureRareData().m_inlineBoxWrapper = boxWrapper;
 }
 
 } // namespace blink
 
 #endif // LayoutBox_h