Bug 642257. If there are no plugins in the display list skip a potentially expensive compute visibility pass with accurate visible regions to determine plugin geometry because it is not needed. r=roc, a=bajaj

If there is a plugin in a background tab and a complex scene in a foreground tab the accurate visible regions can cause us to bog down, for no good reason.

layout/base/nsDisplayList.cpp

@@ -448,7 +448,8 @@ nsDisplayListBuilder::nsDisplayListBuilder(nsIFrame* aReferenceFrame,
       mHasDisplayPort(false),
       mHasFixedItems(false),
       mIsInFixedPosition(false),
-      mIsCompositingCheap(false)
+      mIsCompositingCheap(false),
+      mContainsPluginItem(false)
 {
   MOZ_COUNT_CTOR(nsDisplayListBuilder);
   PL_InitArenaPool(&mPool, "displayListArena", 1024,

layout/base/nsDisplayList.h

@@ -582,6 +582,9 @@ public:
     return aItem == mGlassDisplayItem;
   }
 
+  void SetContainsPluginItem() { mContainsPluginItem = true; }
+  bool ContainsPluginItem() { return mContainsPluginItem; }
+
 private:
   void MarkOutOfFlowFrameForDisplay(nsIFrame* aDirtyFrame, nsIFrame* aFrame,
                                     const nsRect& aDirtyRect);
@@ -635,6 +638,7 @@ private:
   bool                           mHasFixedItems;
   bool                           mIsInFixedPosition;
   bool                           mIsCompositingCheap;
+  bool                           mContainsPluginItem;
 };
 
 class nsDisplayItem;

layout/base/nsPresContext.cpp

@@ -2497,10 +2497,8 @@ nsRootPresContext::ComputePluginGeometryUpdates(nsIFrame* aFrame,
   mRegisteredPlugins.EnumerateEntries(SetPluginHidden, aFrame);
 
   nsIFrame* rootFrame = FrameManager()->GetRootFrame();
-  if (!rootFrame) {
-    return;
-  }
 
+  if (rootFrame && aBuilder->ContainsPluginItem()) {
     aBuilder->SetForPluginGeometry();
     aBuilder->SetAccurateVisibleRegions();
     // Merging and flattening has already been done and we should not do it
@@ -2511,6 +2509,7 @@ nsRootPresContext::ComputePluginGeometryUpdates(nsIFrame* aFrame,
     // nsDisplayPlugin::ComputeVisibility will automatically set a non-hidden
     // widget configuration for the plugin, if it's visible.
     aList->ComputeVisibilityForRoot(aBuilder, &region);
+  }
 
   InitApplyPluginGeometryTimer();
 }

layout/generic/nsObjectFrame.h

@@ -310,6 +310,7 @@ public:
     : nsDisplayItem(aBuilder, aFrame)
   {
     MOZ_COUNT_CTOR(nsDisplayPlugin);
+    aBuilder->SetContainsPluginItem();
   }
 #ifdef NS_BUILD_REFCNT_LOGGING
   virtual ~nsDisplayPlugin() {