Bug 1021085 - Refactor code to ensure we use the same scrolling ancestor layer everywhere. r=Cwiiis

gfx/layers/client/ClientTiledThebesLayer.cpp

@@ -85,6 +85,32 @@ GetTransformToAncestorsParentLayer(Layer* aStart, Layer* aAncestor)
   return ret;
 }
 
+void
+ClientTiledThebesLayer::GetAncestorLayers(ContainerLayer** aOutScrollAncestor,
+                                          ContainerLayer** aOutDisplayPortAncestor)
+{
+  ContainerLayer* scrollAncestor = nullptr;
+  ContainerLayer* displayPortAncestor = nullptr;
+  for (ContainerLayer* ancestor = GetParent(); ancestor; ancestor = ancestor->GetParent()) {
+    const FrameMetrics& metrics = ancestor->GetFrameMetrics();
+    if (!scrollAncestor && metrics.GetScrollId() != FrameMetrics::NULL_SCROLL_ID) {
+      scrollAncestor = ancestor;
+    }
+    if (!metrics.mDisplayPort.IsEmpty()) {
+      displayPortAncestor = ancestor;
+      // Any layer that has a displayport must be scrollable, so we can break
+      // here.
+      break;
+    }
+  }
+  if (aOutScrollAncestor) {
+    *aOutScrollAncestor = scrollAncestor;
+  }
+  if (aOutDisplayPortAncestor) {
+    *aOutDisplayPortAncestor = displayPortAncestor;
+  }
+}
+
 void
 ClientTiledThebesLayer::BeginPaint()
 {
@@ -104,18 +130,7 @@ ClientTiledThebesLayer::BeginPaint()
   // with a displayport.
   ContainerLayer* scrollAncestor = nullptr;
   ContainerLayer* displayPortAncestor = nullptr;
-  for (ContainerLayer* ancestor = GetParent(); ancestor; ancestor = ancestor->GetParent()) {
-    const FrameMetrics& metrics = ancestor->GetFrameMetrics();
-    if (!scrollAncestor && metrics.GetScrollId() != FrameMetrics::NULL_SCROLL_ID) {
-      scrollAncestor = ancestor;
-    }
-    if (!metrics.mDisplayPort.IsEmpty()) {
-      displayPortAncestor = ancestor;
-      // Any layer that has a displayport must be scrollable, so we can break
-      // here.
-      break;
-    }
-  }
+  GetAncestorLayers(&scrollAncestor, &displayPortAncestor);
 
   if (!displayPortAncestor || !scrollAncestor) {
     // No displayport or scroll ancestor, so we can't do progressive rendering.

gfx/layers/client/ClientTiledThebesLayer.h

@@ -69,6 +69,14 @@ public:
 
   virtual void ClearCachedResources() MOZ_OVERRIDE;
 
+  /**
+   * Helper method to find the nearest ancestor layers which
+   * scroll and have a displayport. The parameters are out-params
+   * which hold the return values; the values passed in may be null.
+   */
+  void GetAncestorLayers(ContainerLayer** aOutScrollAncestor,
+                         ContainerLayer** aOutDisplayPortAncestor);
+
 private:
   ClientLayerManager* ClientManager()
   {

gfx/layers/client/TiledContentClient.cpp

@@ -1011,11 +1011,12 @@ ClientTiledLayerBuffer::ComputeProgressiveUpdateRegion(const nsIntRegion& aInval
 #else
   MOZ_ASSERT(mSharedFrameMetricsHelper);
 
-  ContainerLayer* parent = mThebesLayer->AsLayer()->GetParent();
+  ContainerLayer* scrollAncestor = nullptr;
+  mThebesLayer->GetAncestorLayers(&scrollAncestor, nullptr);
 
   bool abortPaint =
     mSharedFrameMetricsHelper->UpdateFromCompositorFrameMetrics(
-      parent,
+      scrollAncestor,
       !staleRegion.Contains(aInvalidRegion),
       drawingLowPrecision,
       compositionBounds,