Bug 975931. Part 8: When we reduce a layer's shadow-visible-region due to opaque covering content, restore it after compositing to ensure it's correct if the opaque content isn't there next time. r=mattwoodrow

--HG--
extra : rebase_source : 72119bd17b0606af70b4f6dbf05cea631aa71e9a

gfx/layers/composite/ContainerLayerComposite.cpp

@@ -311,6 +311,14 @@ ContainerRender(ContainerT* aContainer,
       continue;
     }
 
+    nsIntRect clipRect = layerToRender->GetLayer()->
+        CalculateScissorRect(aClipRect, &aManager->GetWorldTransform());
+    if (clipRect.IsEmpty()) {
+      continue;
+    }
+
+    nsIntRegion savedVisibleRegion;
+    bool restoreVisibleRegion = false;
     if (i + 1 < children.Length() &&
         layerToRender->GetLayer()->GetEffectiveTransform().IsIdentity()) {
       LayerComposite* nextLayer = static_cast<LayerComposite*>(children.ElementAt(i + 1)->ImplData());
@@ -319,21 +327,17 @@ ContainerRender(ContainerT* aContainer,
         nextLayerOpaqueRect = GetOpaqueRect(nextLayer->GetLayer());
       }
       if (!nextLayerOpaqueRect.IsEmpty()) {
+        savedVisibleRegion = layerToRender->GetShadowVisibleRegion();
         nsIntRegion visibleRegion;
-        visibleRegion.Sub(layerToRender->GetShadowVisibleRegion(), nextLayerOpaqueRect);
-        layerToRender->SetShadowVisibleRegion(visibleRegion);
+        visibleRegion.Sub(savedVisibleRegion, nextLayerOpaqueRect);
         if (visibleRegion.IsEmpty()) {
           continue;
         }
+        layerToRender->SetShadowVisibleRegion(visibleRegion);
+        restoreVisibleRegion = true;
       }
     }
 
-    nsIntRect clipRect = layerToRender->GetLayer()->
-        CalculateScissorRect(aClipRect, &aManager->GetWorldTransform());
-    if (clipRect.IsEmpty()) {
-      continue;
-    }
-
     if (layerToRender->HasLayerBeenComposited()) {
       // Composer2D will compose this layer so skip GPU composition
       // this time & reset composition flag for next composition phase
@@ -349,6 +353,11 @@ ContainerRender(ContainerT* aContainer,
       layerToRender->RenderLayer(clipRect);
     }
 
+    if (restoreVisibleRegion) {
+      // Restore the region in case it's not covered by opaque content next time
+      layerToRender->SetShadowVisibleRegion(savedVisibleRegion);
+    }
+
     if (gfxPrefs::LayersScrollGraph()) {
       DrawVelGraph(clipRect, aManager, layerToRender->GetLayer());
     }