Bug 728284 - Main container layer has siblings that are drawn in the background. r=bgirard

gfx/layers/ipc/CompositorParent.cpp

@@ -221,6 +221,23 @@ CompositorParent::Composite()
 #endif
 }
 
+// Go down shadow layer tree, setting properties to match their non-shadow
+// counterparts.
+static void
+SetShadowProperties(Layer* aLayer)
+{
+  // FIXME: Bug 717688 -- Do these updates in ShadowLayersParent::RecvUpdate.
+  ShadowLayer* shadow = aLayer->AsShadowLayer();
+  shadow->SetShadowTransform(aLayer->GetTransform());
+  shadow->SetShadowVisibleRegion(aLayer->GetVisibleRegion());
+  shadow->SetShadowClipRect(aLayer->GetClipRect());
+
+  for (Layer* child = aLayer->GetFirstChild();
+      child; child = child->GetNextSibling()) {
+    SetShadowProperties(child);
+  }
+}
+
 #ifdef MOZ_WIDGET_ANDROID
 // Do a breadth-first search to find the first layer in the tree that is
 // scrollable.
@@ -229,6 +246,21 @@ CompositorParent::GetPrimaryScrollableLayer()
 {
   Layer* root = mLayerManager->GetRoot();
 
+  // FIXME: We're currently getting passed layers that are not part of our content, but
+  // we are drawing them anyway. This is causing severe rendering corruption to our background
+  // and checkerboarding. The real fix here is to assert that we don't have any useless layers
+  // and ensure that layout isn't giving us any. This is being tracked in bug 728284.
+  // For now just clip them to the empty rect so we don't draw them.
+  Layer* discardLayer = root->GetFirstChild();
+
+  while (discardLayer) {
+    if (!discardLayer->AsContainerLayer()) {
+      discardLayer->IntersectClipRect(nsIntRect());
+      SetShadowProperties(discardLayer);
+    }
+    discardLayer = discardLayer->GetNextSibling();
+  }
+
   nsTArray<Layer*> queue;
   queue.AppendElement(root);
   while (queue.Length()) {
@@ -254,23 +286,6 @@ CompositorParent::GetPrimaryScrollableLayer()
 }
 #endif
 
-// Go down shadow layer tree, setting properties to match their non-shadow
-// counterparts.
-static void
-SetShadowProperties(Layer* aLayer)
-{
-  // FIXME: Bug 717688 -- Do these updates in ShadowLayersParent::RecvUpdate.
-  ShadowLayer* shadow = aLayer->AsShadowLayer();
-  shadow->SetShadowTransform(aLayer->GetTransform());
-  shadow->SetShadowVisibleRegion(aLayer->GetVisibleRegion());
-  shadow->SetShadowClipRect(aLayer->GetClipRect());
-
-  for (Layer* child = aLayer->GetFirstChild();
-      child; child = child->GetNextSibling()) {
-    SetShadowProperties(child);
-  }
-}
-
 void
 CompositorParent::TransformShadowTree()
 {