3 files changed, 18 insertions, 8 deletions

diff --git a/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp b/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
index d370f33..ab0e0ea 100644
--- a/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
+++ b/WebCore/platform/graphics/android/GraphicsLayerAndroid.cpp
@@ -257,10 +257,11 @@ void GraphicsLayerAndroid::updateFixedPosition()
 
         SkRect viewRect;
         viewRect.set(paintingOffsetX, paintingOffsetY, paintingOffsetX + w, paintingOffsetY + h);
-
+        IntPoint renderLayerPos(renderLayer->x(), renderLayer->y());
         m_contentLayer->setFixedPosition(left, top, right, bottom,
                                          marginLeft, marginTop,
                                          marginRight, marginBottom,
+                                         renderLayerPos,
                                          viewRect);
     }
 }
diff --git a/WebCore/platform/graphics/android/LayerAndroid.cpp b/WebCore/platform/graphics/android/LayerAndroid.cpp
index 6426716..2f55292 100644
--- a/WebCore/platform/graphics/android/LayerAndroid.cpp
+++ b/WebCore/platform/graphics/android/LayerAndroid.cpp
@@ -101,7 +101,7 @@ LayerAndroid::LayerAndroid(const LayerAndroid& layer) : SkLayer(layer),
     m_isFixed = layer.m_isFixed;
     m_contentsImage = layer.m_contentsImage;
     m_contentsImage->safeRef();
-
+    m_renderLayerPos = layer.m_renderLayerPos;
     m_transform = layer.m_transform;
     m_backgroundColor = layer.m_backgroundColor;
 
@@ -462,17 +462,20 @@ void LayerAndroid::updateFixedLayersPositions(const SkRect& viewport)
         float x = dx;
         float y = dy;
 
-        // Not defined corresponds to 'auto';
-        // If both left and right are auto, the w3c says we should set left
-        // to zero (in left-to-right layout). So we use left if it's defined
-        // or if right isn't defined.
+        // It turns out that when it is 'auto', the webkit computation will
+        // take one more factor into account,  that is the original render
+        // layer's X,Y, such that it will align well with the parent's layer.
+        if (!(m_fixedLeft.defined() || m_fixedRight.defined()))
+            x += m_renderLayerPos.x();
+
+        if (!(m_fixedTop.defined() || m_fixedBottom.defined()))
+            y += m_renderLayerPos.y();
+
         if (m_fixedLeft.defined() || !m_fixedRight.defined())
             x += m_fixedMarginLeft.calcFloatValue(w) + m_fixedLeft.calcFloatValue(w) - m_fixedRect.fLeft;
         else
             x += w - m_fixedMarginRight.calcFloatValue(w) - m_fixedRight.calcFloatValue(w) - m_fixedRect.fRight;
 
-        // Following the same reason as above, if bottom isn't defined, we apply
-        // top regardless of it being defined or not.
         if (m_fixedTop.defined() || !m_fixedBottom.defined())
             y += m_fixedMarginTop.calcFloatValue(h) + m_fixedTop.calcFloatValue(h) - m_fixedRect.fTop;
         else
diff --git a/WebCore/platform/graphics/android/LayerAndroid.h b/WebCore/platform/graphics/android/LayerAndroid.h
index 72eba0f..bb91755 100644
--- a/WebCore/platform/graphics/android/LayerAndroid.h
+++ b/WebCore/platform/graphics/android/LayerAndroid.h
@@ -138,6 +138,7 @@ public:
                           SkLength marginTop, // CSS margin-top property
                           SkLength marginRight, // CSS margin-right property
                           SkLength marginBottom, // CSS margin-bottom property
+                          const IntPoint& renderLayerPos, // For undefined fixed position
                           SkRect viewRect) { // view rect, can be smaller than the layer's
         m_fixedLeft = left;
         m_fixedTop = top;
@@ -149,6 +150,7 @@ public:
         m_fixedMarginBottom = marginBottom;
         m_fixedRect = viewRect;
         m_isFixed = true;
+        m_renderLayerPos = renderLayerPos;
         setInheritFromRootTransform(true);
     }
 
@@ -259,6 +261,10 @@ private:
     SkLength m_fixedMarginBottom;
     SkRect m_fixedRect;
 
+    // When fixed element is undefined or auto, the render layer's position
+    // is needed for offset computation
+    IntPoint m_renderLayerPos;
+
     TransformationMatrix m_transform;
 
     SkColor m_backgroundColor;