Backout 4c89002460da (Bug 873378) for breaking the settings app on B2G

layout/base/FrameLayerBuilder.cpp

@@ -2041,9 +2041,6 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
     topLeft = lastActiveScrolledRoot->GetOffsetToCrossDoc(mContainerReferenceFrame);
   }
 
-  int32_t maxLayers = nsDisplayItem::MaxActiveLayers();
-  int layerCount = 0;
-
   for (nsDisplayItem* item = aList.GetBottom(); item; item = item->GetAbove()) {
     NS_ASSERTION(mAppUnitsPerDevPixel == AppUnitsPerDevPixel(item),
       "items in a container layer should all have the same app units per dev pixel");
@@ -2086,10 +2083,6 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
       }
     }
 
-    if (maxLayers != -1 && layerCount >= maxLayers) {
-      forceInactive = true;
-    }
-
     // Assign the item to a layer
     if (layerState == LAYER_ACTIVE_FORCE ||
         (layerState == LAYER_INACTIVE && !mManager->IsWidgetLayerManager()) ||
@@ -2097,8 +2090,6 @@ ContainerState::ProcessDisplayItems(const nsDisplayList& aList,
          (layerState == LAYER_ACTIVE_EMPTY ||
           layerState == LAYER_ACTIVE))) {
 
-      layerCount++;
-
       // LAYER_ACTIVE_EMPTY means the layer is created just for its metadata.
       // We should never see an empty layer with any visible content!
       NS_ASSERTION(layerState != LAYER_ACTIVE_EMPTY ||

layout/base/nsDisplayList.cpp

@@ -1497,20 +1497,6 @@ nsDisplayItem::ForceActiveLayers()
   return sForce;
 }
 
-/* static */ int32_t
-nsDisplayItem::MaxActiveLayers()
-{
-  static int32_t sMaxLayers = false;
-  static bool sMaxLayersCached = false;
-
-  if (!sMaxLayersCached) {
-    Preferences::AddIntVarCache(&sMaxLayers, "layers.max-active", -1);
-    sMaxLayersCached = true;
-  }
-
-  return sMaxLayersCached;
-}
-
 bool
 nsDisplayItem::RecomputeVisibility(nsDisplayListBuilder* aBuilder,
                                    nsRegion* aVisibleRegion) {

layout/base/nsDisplayList.h

@@ -1003,12 +1003,6 @@ public:
    */
   static bool ForceActiveLayers();
 
-  /**
-   * Returns the maximum number of layers that should be created
-   * or -1 for no limit. Requires setting the pref layers.max-acitve.
-   */
-  static int32_t MaxActiveLayers();
-
   /**
    * @return LAYER_NONE if BuildLayer will return null. In this case
    * there is no layer for the item, and Paint should be called instead

mobile/android/app/mobile.js

@@ -546,12 +546,6 @@ pref("layers.async-video.enabled", true);
 pref("layers.progressive-paint", true);
 pref("layers.low-precision-buffer", true);
 pref("layers.low-precision-resolution", 250);
-// We want to limit layers for two reasons:
-// 1) We can't scroll smoothly if we have to many draw calls
-// 2) Pages that have too many layers consume too much memory and crash.
-// By limiting the number of layers on mobile we're making the main thread
-// work harder keep scrolling smooth and memory low.
-pref("layers.max-active", 20);
 
 pref("notification.feature.enabled", true);
 pref("dom.webnotifications.enabled", true);

modules/libpref/src/init/all.js

@@ -4062,10 +4062,6 @@ pref("layers.acceleration.draw-fps", false);
 pref("layers.draw-borders", false);
 pref("layers.frame-counter", false);
 
-// Max number of layers per container. See Overwrite in mobile prefs.
-pref("layers.max-active", -1);
-
-
 #ifdef XP_MACOSX
 pref("layers.offmainthreadcomposition.enabled", true);
 #else