Bug 867770 - Add a pref as to whether we layerize animated images, disabled by default. r=mattwoodrow

layout/base/nsDisplayList.cpp

@@ -1846,6 +1846,7 @@ nsDisplayBackgroundImage::GetLayerState(nsDisplayListBuilder* aBuilder,
                                         const FrameLayerBuilder::ContainerParameters& aParameters)
 {
   if (!TryOptimizeToImageLayer(aManager, aBuilder) ||
+      !nsLayoutUtils::AnimatedImageLayersEnabled() ||
       !mIsAnimated) {
     if (!aManager->IsCompositingCheap() ||
         !nsLayoutUtils::GPUImageScalingEnabled()) {

layout/base/nsLayoutUtils.cpp

@@ -382,6 +382,22 @@ nsLayoutUtils::GPUImageScalingEnabled()
   return sGPUImageScalingEnabled;
 }
 
+bool
+nsLayoutUtils::AnimatedImageLayersEnabled()
+{
+  static bool sAnimatedImageLayersEnabled;
+  static bool sAnimatedImageLayersPrefCached = false;
+
+  if (!sAnimatedImageLayersPrefCached) {
+    sAnimatedImageLayersPrefCached = true;
+    Preferences::AddBoolVarCache(&sAnimatedImageLayersEnabled,
+                                 "layout.animated-image-layers.enabled",
+                                 false);
+  }
+
+  return sAnimatedImageLayersEnabled;
+}
+
 void
 nsLayoutUtils::UnionChildOverflow(nsIFrame* aFrame,
                                   nsOverflowAreas& aOverflowAreas)

layout/base/nsLayoutUtils.h

@@ -1613,6 +1613,11 @@ public:
    */
   static bool GPUImageScalingEnabled();
 
+  /**
+   * Checks whether we want to layerize animated images whenever possible.
+   */
+  static bool AnimatedImageLayersEnabled();
+
   /**
    * Unions the overflow areas of all non-popup children of aFrame with
    * aOverflowAreas.

layout/generic/nsImageFrame.cpp

@@ -1259,7 +1259,8 @@ nsDisplayImage::GetLayerState(nsDisplayListBuilder* aBuilder,
                               const FrameLayerBuilder::ContainerParameters& aParameters)
 {
   bool animated = false;
-  if (mImage->GetType() != imgIContainer::TYPE_RASTER ||
+  if (!nsLayoutUtils::AnimatedImageLayersEnabled() ||
+      mImage->GetType() != imgIContainer::TYPE_RASTER ||
       NS_FAILED(mImage->GetAnimated(&animated)) ||
       !animated) {
     if (!aManager->IsCompositingCheap() ||

modules/libpref/src/init/all.js

@@ -4078,6 +4078,9 @@ pref("dom.event.handling-user-input-time-limit", 1000);
 //3D Transforms
 pref("layout.3d-transforms.enabled", true);
 
+// Whether we should layerize all animated images (if otherwise possible).
+pref("layout.animated-image-layers.enabled", false);
+
 pref("dom.vibrator.enabled", true);
 pref("dom.vibrator.max_vibrate_ms", 10000);
 pref("dom.vibrator.max_vibrate_list_len", 128);