Bug 846901 - Support prerendering elements with animated transforms that are offscreen but only by a small amount. r=mattwoodrow

layout/base/FrameLayerBuilder.cpp

@@ -2136,7 +2136,8 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
       // Note that items without their own layers can't be skipped this
       // way, since their ThebesLayer may decide it wants to draw them
       // into its buffer even if they're currently covered.
-      if (itemVisibleRect.IsEmpty() && layerState != LAYER_ACTIVE_EMPTY) {
+      if (itemVisibleRect.IsEmpty() &&
+          !item->ShouldBuildLayerEvenIfInvisible(mBuilder)) {
         continue;
       }
 
@@ -2192,7 +2193,9 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
         data->mDrawAboveRegion.SimplifyOutward(4);
       }
       itemVisibleRect.MoveBy(mParameters.mOffset);
+      if (!nsDisplayTransform::IsLayerPrerendered(ownLayer)) {
         RestrictVisibleRegionForLayer(ownLayer, itemVisibleRect);
+      }
 
       // rounded rectangle clipping using mask layers
       // (must be done after visible rect is set on layer)

layout/base/nsDisplayList.cpp

@@ -570,12 +570,24 @@ void nsDisplayListBuilder::MarkOutOfFlowFrameForDisplay(nsIFrame* aDirtyFrame,
   nsRect dirty = aDirtyRect - aFrame->GetOffsetTo(aDirtyFrame);
   nsRect overflowRect = aFrame->GetVisualOverflowRect();
 
+  if (aFrame->IsTransformed() &&
+      nsLayoutUtils::HasAnimationsForCompositor(aFrame->GetContent(),
+                                                eCSSProperty_transform)) {
+   /**
+    * Add a fuzz factor to the overflow rectangle so that elements only just
+    * out of view are pulled into the display list, so they can be
+    * prerendered if necessary.
+    */
+    overflowRect.Inflate(nsPresContext::CSSPixelsToAppUnits(32));
+  }
+
   if (mHasDisplayPort && IsFixedFrame(aFrame)) {
     dirty = overflowRect;
   }
 
   if (!dirty.IntersectRect(dirty, overflowRect))
     return;
+
   aFrame->Properties().Set(nsDisplayListBuilder::OutOfFlowDirtyRectProperty(),
                            new nsRect(dirty));
 
@@ -3891,7 +3903,14 @@ nsDisplayTransform::ShouldPrerenderTransformedContent(nsDisplayListBuilder* aBui
                                                       nsIFrame* aFrame,
                                                       bool aLogAnimations)
 {
-  if (!aFrame->AreLayersMarkedActive(nsChangeHint_UpdateTransformLayer)) {
+  // Elements whose transform has been modified recently, or which
+  // have a compositor-animated transform, can be prerendered. An element
+  // might have only just had its transform animated in which case
+  // nsChangeHint_UpdateTransformLayer will not be present yet.
+  if (!aFrame->AreLayersMarkedActive(nsChangeHint_UpdateTransformLayer) &&
+      (!aFrame->GetContent() ||
+       !nsLayoutUtils::HasAnimationsForCompositor(aFrame->GetContent(),
+                                                  eCSSProperty_transform))) {
     if (aLogAnimations) {
       nsCString message;
       message.AppendLiteral("Performance warning: Async animation disabled because frame was not marked active for transform animation");
@@ -3978,6 +3997,12 @@ nsDisplayTransform::GetTransform(float aAppUnitsPerPixel)
   return mTransform;
 }
 
+bool
+nsDisplayTransform::ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder)
+{
+  return ShouldPrerenderTransformedContent(aBuilder, mFrame, false);
+}
+
 already_AddRefed<Layer> nsDisplayTransform::BuildLayer(nsDisplayListBuilder *aBuilder,
                                                        LayerManager *aManager,
                                                        const ContainerParameters& aContainerParameters)

layout/base/nsDisplayList.h

@@ -966,6 +966,12 @@ public:
                                    LayerManager* aManager,
                                    const ContainerParameters& aParameters)
   { return mozilla::LAYER_NONE; }
+  /**
+   * Return true to indicate the layer should be constructed even if it's
+   * completely invisible.
+   */
+  virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder)
+  { return false; }
   /**
    * Actually paint this item to some rendering context.
    * Content outside mVisibleRect need not be painted.
@@ -2533,7 +2539,8 @@ public:
   virtual LayerState GetLayerState(nsDisplayListBuilder* aBuilder,
                                    LayerManager* aManager,
                                    const ContainerParameters& aParameters) MOZ_OVERRIDE;
-
+  virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder) MOZ_OVERRIDE
+  { return true; }
   virtual bool TryMerge(nsDisplayListBuilder* aBuilder,
                           nsDisplayItem* aItem) MOZ_OVERRIDE;
 
@@ -2789,6 +2796,7 @@ public:
   virtual already_AddRefed<Layer> BuildLayer(nsDisplayListBuilder* aBuilder,
                                              LayerManager* aManager,
                                              const ContainerParameters& aContainerParameters) MOZ_OVERRIDE;
+  virtual bool ShouldBuildLayerEvenIfInvisible(nsDisplayListBuilder* aBuilder) MOZ_OVERRIDE;
   virtual bool ComputeVisibility(nsDisplayListBuilder *aBuilder,
                                    nsRegion *aVisibleRegion,
                                    const nsRect& aAllowVisibleRegionExpansion) MOZ_OVERRIDE;
@@ -2940,6 +2948,11 @@ public:
                                                 bool aLogAnimations = false);
   bool CanUseAsyncAnimations(nsDisplayListBuilder* aBuilder) MOZ_OVERRIDE;
 
+  static bool IsLayerPrerendered(Layer* aLayer)
+  {
+    return aLayer->HasUserData(nsIFrame::LayerIsPrerenderedDataKey());
+  }
+
 private:
   static gfx3DMatrix GetResultingTransformMatrixInternal(const FrameTransformProperties& aProperties,
                                                          const nsPoint& aOrigin,

layout/base/nsLayoutDebugger.cpp

@@ -130,10 +130,6 @@ PrintDisplayListTo(nsDisplayListBuilder* aBuilder, const nsDisplayList& aList,
   }
 
   for (nsDisplayItem* i = aList.GetBottom(); i != nullptr; i = i->GetAbove()) {
-#ifdef DEBUG
-    if (aList.DidComputeVisibility() && i->GetVisibleRect().IsEmpty())
-      continue;
-#endif
     if (aDumpHtml) {
       fprintf(aOutput, "<li>");
     } else {