(WIP) Paint invalidation for slimming paint v2

Source/core/layout/LayoutObject.h

Index: Source/core/layout/LayoutObject.h
diff --git a/Source/core/layout/LayoutObject.h b/Source/core/layout/LayoutObject.h
index 84e8af67fea41dae10f28f58c5ebb7ae6fe14f0d..296b8bfcc7fc582684610e4c5a84f59cfc6f4d9c 100644
--- a/Source/core/layout/LayoutObject.h
+++ b/Source/core/layout/LayoutObject.h
@@ -913,6 +913,7 @@ public:
     inline void setSelectionStateIfNeeded(SelectionState);
     bool canUpdateSelectionOnRootLineBoxes() const;
 
+    // TODO: Remove for slimming paint v2.
     // A single rectangle that encompasses all of the selected objects within this object.  Used to determine the tightest
     // possible bounding box for the selection. The rect returned is in the coordinate space of the paint invalidation container's backing.
     virtual LayoutRect selectionRectForPaintInvalidation(const LayoutBoxModelObject* /*paintInvalidationContainer*/) const { return LayoutRect(); }
@@ -1032,6 +1033,7 @@ public:
     bool shouldInvalidateOverflowForPaint() const { return m_bitfields.shouldInvalidateOverflowForPaint(); }
 
     virtual void clearPaintInvalidationState(const PaintInvalidationState&);
+    void clearSelfPaintInvalidationFlags();
 
     bool mayNeedPaintInvalidation() const { return m_bitfields.mayNeedPaintInvalidation(); }
     void setMayNeedPaintInvalidation();
@@ -1049,7 +1051,12 @@ public:
 
     bool shouldCheckForPaintInvalidationRegardlessOfPaintInvalidationState() const
     {
-        return mayNeedPaintInvalidation() || shouldDoFullPaintInvalidation() || shouldInvalidateSelection() || m_bitfields.childShouldCheckForPaintInvalidation();
+        return selfShouldCheckForPaintInvalidation() || m_bitfields.childShouldCheckForPaintInvalidation();
+    }
+
+    bool selfShouldCheckForPaintInvalidation() const
+    {
+        return mayNeedPaintInvalidation() || shouldDoFullPaintInvalidation() || shouldInvalidateSelection();
     }
 
     virtual bool supportsPaintInvalidationStateCachedOffsets() const { return !hasTransformRelatedProperty() && !hasReflection() && !style()->isFlippedBlocksWritingMode(); }
@@ -1076,6 +1083,7 @@ public:
     bool childNeedsRepaint() const { return m_bitfields.childNeedsRepaint(); }
     bool shouldCheckForRepaint() const { return selfNeedsRepaint() || childNeedsRepaint(); }
 
+    virtual PaintInvalidationReason invalidatePaintIfNeededForSlimmingPaintV2(const LayoutPoint& newPaintOffset);
     void clearRepaintFlagsRecursively();
 
 protected:
@@ -1246,6 +1254,7 @@ private:
 
     void invalidatePaintIncludingNonCompositingDescendantsInternal(const LayoutBoxModelObject& repaintContainer);
 
+    // TODO: Remove for slimming paint v2.
     LayoutRect previousSelectionRectForPaintInvalidation() const;
     void setPreviousSelectionRectForPaintInvalidation(const LayoutRect&);
 
@@ -1352,7 +1361,7 @@ private:
         {
         }
 
-        // 32 bits have been used in the first word, and 18 in the second.
+        // 32 bits have been used in the first word, and 19 in the second.
         ADD_BOOLEAN_BITFIELD(selfNeedsLayout, SelfNeedsLayout);
         ADD_BOOLEAN_BITFIELD(shouldInvalidateOverflowForPaint, ShouldInvalidateOverflowForPaint); // TODO(wangxianzhu): Remove for slimming paint v2.
         ADD_BOOLEAN_BITFIELD(childShouldCheckForPaintInvalidation, ChildShouldCheckForPaintInvalidation);
@@ -1461,12 +1470,17 @@ private:
 
     // This stores the paint invalidation rect from the previous frame. This rect does *not* account for composited scrolling. See
     // adjustInvalidationRectForCompositedScrolling().
+    // TODO: Remove for slimming paint v2.
     LayoutRect m_previousPaintInvalidationRect;
 
     // This stores the position in the paint invalidation backing's coordinate.
     // It is used to detect layoutObject shifts that forces a full invalidation.
     // This point does *not* account for composited scrolling. See adjustInvalidationRectForCompositedScrolling().
-    LayoutPoint m_previousPositionFromPaintInvalidationBacking;
+    union {
+        LayoutPoint m_previousPositionFromPaintInvalidationBacking; // TODO: Remove for slimming paint v2.
+        // For slimming paint v2: previous paint offset in the space of the nearest transform container.
+        LayoutPoint m_previousPaintOffset;
+    };
 };
 
 // FIXME: remove this once the layout object lifecycle ASSERTS are no longer hit.