Generate scroll/clip display item hierarchy for SPv2

Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp

Index: Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp
diff --git a/Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp b/Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp
index 6689f124037802d870a86390a1182af0ef09209d..b444544da138fea75b3aa6fcbc52c189c7f3e4f5 100644
--- a/Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp
+++ b/Source/platform/graphics/paint/DisplayItemPropertyTreeBuilder.cpp
@@ -5,9 +5,11 @@
 #include "config.h"
 #include "platform/graphics/paint/DisplayItemPropertyTreeBuilder.h"
 
+#include "platform/RuntimeEnabledFeatures.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/graphics/paint/DisplayItemClipTree.h"
 #include "platform/graphics/paint/DisplayItemTransformTree.h"
+#include "platform/graphics/paint/FixedPositionDisplayItem.h"
 #include "platform/graphics/paint/ScrollDisplayItem.h"
 #include "platform/graphics/paint/Transform3DDisplayItem.h"
 #include "platform/graphics/paint/TransformDisplayItem.h"
@@ -56,6 +58,8 @@ void DisplayItemPropertyTreeBuilder::processDisplayItem(const DisplayItem& displ
         processBeginItem(displayItem);
     else if (displayItem.isEnd())
         processEndItem(displayItem);
+    else if (displayItem.type() == DisplayItem::FixedPositionContainer)
+        processAnchorItem(displayItem);
     m_currentIndex++;
 }
 
@@ -63,12 +67,15 @@ namespace {
 
 enum TransformKind { NotATransform = 0, Only2DTranslation, RequiresTransformNode };
 enum ClipKind { NotAClip = 0, RequiresClipNode };
+enum SpecialKind { NotSpecial = 0, IsFixedPosition };
 
 struct BeginDisplayItemClassification {
     TransformKind transformKind = NotATransform;
     TransformationMatrix matrix;
     ClipKind clipKind = NotAClip;
     FloatRect clipRect;
+    SpecialKind specialKind = NotSpecial;
+    DisplayItemClient anchorKey;
 };
 
 // Classifies a begin display item based on how it affects the property trees.
@@ -91,6 +98,9 @@ static BeginDisplayItemClassification classifyBeginItem(const DisplayItem& begin
         const IntSize& offset = beginScroll.currentOffset();
         result.matrix.translate(-offset.width(), -offset.height());
         result.transformKind = Only2DTranslation;
+    } else if (type == DisplayItem::BeginFixedPosition) {
+        result.specialKind = IsFixedPosition;
+        result.anchorKey = static_cast<const BeginFixedPositionDisplayItem&>(beginDisplayItem).anchor();
     }
     return result;
 }
@@ -101,7 +111,7 @@ void DisplayItemPropertyTreeBuilder::processBeginItem(const DisplayItem& display
 {
     BeginDisplayItemClassification classification = classifyBeginItem(displayItem);
 
-    if (!classification.transformKind && !classification.clipKind) {
+    if (!classification.transformKind && !classification.clipKind && !classification.specialKind) {
         currentState().ignoredBeginCount++;
         return;
     }
@@ -137,6 +147,23 @@ void DisplayItemPropertyTreeBuilder::processBeginItem(const DisplayItem& display
         break;
     }
 
+    switch (classification.specialKind) {
+    case NotSpecial:
+        break;
+    case IsFixedPosition:
+        BuilderState referredState(m_anchors.get(classification.anchorKey));
+        newState.transformNode = referredState.transformNode;
+        newState.offset = referredState.offset;
+        newState.clipNode = referredState.clipNode;
+        {
+            // Note: This is only temporarily workaround before we implement true
+            // transform-tree-based rasterization.
+            ASSERT(currentState().transformNode == newState.transformNode);
+            static_cast<const BeginFixedPositionDisplayItem&>(displayItem).setCounterScroll(newState.offset - currentState().offset);
+        }
+        break;
+    }
+
     m_stateStack.append(newState);
 }
 
@@ -153,6 +180,14 @@ void DisplayItemPropertyTreeBuilder::processEndItem(const DisplayItem& displayIt
     }
 }
 
+void DisplayItemPropertyTreeBuilder::processAnchorItem(const DisplayItem& displayItem)
+{
+    ASSERT(RuntimeEnabledFeatures::slimmingPaintV2Enabled());
+    // Anchors can duplicate when we have reflection or fragments.
+    // We can't assert uniqueness here. However we may assume the most recent one is valid.
+    m_anchors.set(displayItem.client(), currentState());
+}
+
 void DisplayItemPropertyTreeBuilder::finishRange()
 {
     // Don't emit an empty range record.