Unify code paths for fixed and absolutely positioned objects in accumulateOffsetTowardsAncestor().

Source/core/paint/DeprecatedPaintLayer.cpp

Index: Source/core/paint/DeprecatedPaintLayer.cpp
diff --git a/Source/core/paint/DeprecatedPaintLayer.cpp b/Source/core/paint/DeprecatedPaintLayer.cpp
index 4cc659ddced37bb39eb93558e23121fe4a33aed4..6426049a39d0ab046e34caf29438f411e5c4a869 100644
--- a/Source/core/paint/DeprecatedPaintLayer.cpp
+++ b/Source/core/paint/DeprecatedPaintLayer.cpp
@@ -1250,58 +1250,14 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
         return ancestorLayer;
     }
 
-    if (position == FixedPosition) {
-        // For a fixed layers, we need to walk up to the root to see if there's a fixed position container
-        // (e.g. a transformed layer). It's an error to call convertToLayerCoords() across a layer with a transform,
-        // so we should always find the ancestor at or before we find the fixed position container, if
-        // the container is transformed.
-        DeprecatedPaintLayer* fixedPositionContainerLayer = 0;
-        bool foundAncestor = false;
-        for (DeprecatedPaintLayer* currLayer = layer->parent(); currLayer; currLayer = currLayer->parent()) {
-            if (currLayer == ancestorLayer)
-                foundAncestor = true;
-
-            if (isFixedPositionedContainer(currLayer)) {
-                fixedPositionContainerLayer = currLayer;
-                // A layer that has a transform-related property but not a
-                // transform still acts as a fixed-position container.
-                // Accumulating offsets across such layers is allowed.
-                if (currLayer->transform())
-                    ASSERT_UNUSED(foundAncestor, foundAncestor);
-                break;
-            }
-        }
-
-        ASSERT(fixedPositionContainerLayer); // We should have hit the LayoutView's layer at least.
-
-        if (fixedPositionContainerLayer != ancestorLayer) {
-            LayoutPoint fixedContainerCoords;
-            layer->convertToLayerCoords(fixedPositionContainerLayer, fixedContainerCoords);
-            location += fixedContainerCoords;
-
-            if (foundAncestor) {
-                LayoutPoint ancestorCoords;
-                ancestorLayer->convertToLayerCoords(fixedPositionContainerLayer, ancestorCoords);
-                location += -ancestorCoords;
-            }
-        } else {
-            // LayoutView has been handled in the first top-level 'if' block above.
-            ASSERT(ancestorLayer != layoutObject->view()->layer());
-            ASSERT(ancestorLayer->hasTransformRelatedProperty());
-
-            location += layer->location();
-        }
-        return foundAncestor ? ancestorLayer : fixedPositionContainerLayer;
-    }
-
     DeprecatedPaintLayer* parentLayer;
-    if (position == AbsolutePosition) {
+    if (position == AbsolutePosition || position == FixedPosition) {
         bool foundAncestorFirst;
         parentLayer = layer->enclosingPositionedAncestor(ancestorLayer, &foundAncestorFirst);
 
         if (foundAncestorFirst) {
-            // Found ancestorLayer before the abs. positioned container, so compute offset of both relative
-            // to the positioned ancestor and subtract.
+            // Found ancestorLayer before the container of the out-of-flow object, so compute offset
+            // of both relative to the positioned ancestor and subtract.

[Julien - ping for review, 2015/07/29 23:10:52]

s/positioned ancestor/container (as fixed position elements don't stop at
positioned ancestors).

[mstensho (USE GERRIT), 2015/07/30 09:32:06]

Done.

 
             LayoutPoint thisCoords;
             layer->convertToLayerCoords(parentLayer, thisCoords);
@@ -1322,7 +1278,7 @@ static inline const DeprecatedPaintLayer* accumulateOffsetTowardsAncestor(const
     }
 
     if (!parentLayer)
-        return 0;
+        return nullptr;
 
     location += layer->location();
     return parentLayer;