Bug 735898. Part 3: In DisplayBorderBackgroundOutline, when we have created an nsDisplayBackground item and it's themed, don't bother checking to create an nsDisplayBorder item. r=mattwoodrow

It turns out that calling HasBorder() is especially expensive for themed frames since we call into the theme engine to compute the border, so avoiding it is a nice win.

layout/base/nsDisplayList.h

@@ -1559,6 +1559,9 @@ public:
   virtual nsRect GetBounds(nsDisplayListBuilder* aBuilder, bool* aSnap);
   virtual void Paint(nsDisplayListBuilder* aBuilder, nsRenderingContext* aCtx);
   NS_DISPLAY_DECL_NAME("Background", TYPE_BACKGROUND)
+  // Returns the value of GetUnderlyingFrame()->IsThemed(), but cached
+  bool IsThemed() { return mIsThemed; }
+
 protected:
   nsRegion GetInsideClipRegion(nsPresContext* aPresContext, PRUint8 aClip,
                                const nsRect& aRect, bool* aSnap);

layout/generic/nsFrame.cpp

@@ -1409,16 +1409,19 @@ nsIFrame::HasBorder() const
 nsresult
 nsFrame::DisplayBackgroundUnconditional(nsDisplayListBuilder*   aBuilder,
                                         const nsDisplayListSet& aLists,
-                                        bool                    aForceBackground)
+                                        bool                    aForceBackground,
+                                        nsDisplayBackground**   aBackground)
 {
   // Here we don't try to detect background propagation. Frames that might
   // receive a propagated background should just set aForceBackground to
   // true.
   if (aBuilder->IsForEventDelivery() || aForceBackground ||
       !GetStyleBackground()->IsTransparent() || GetStyleDisplay()->mAppearance) {
-    return aLists.BorderBackground()->AppendNewToTop(new (aBuilder)
-        nsDisplayBackground(aBuilder, this));
+    nsDisplayBackground* bg = new (aBuilder) nsDisplayBackground(aBuilder, this);
+    *aBackground = bg;
+    return aLists.BorderBackground()->AppendNewToTop(bg);
   }
+  *aBackground = nsnull;
   return NS_OK;
 }
 
@@ -1440,8 +1443,9 @@ nsFrame::DisplayBorderBackgroundOutline(nsDisplayListBuilder*   aBuilder,
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
+  nsDisplayBackground* bg;
   nsresult rv =
-    DisplayBackgroundUnconditional(aBuilder, aLists, aForceBackground);
+    DisplayBackgroundUnconditional(aBuilder, aLists, aForceBackground, &bg);
   NS_ENSURE_SUCCESS(rv, rv);
 
   if (hasBoxShadow) {
@@ -1449,8 +1453,11 @@ nsFrame::DisplayBorderBackgroundOutline(nsDisplayListBuilder*   aBuilder,
         nsDisplayBoxShadowInner(aBuilder, this));
     NS_ENSURE_SUCCESS(rv, rv);
   }
-  
-  if (HasBorder()) {
+
+  // If there's a themed background, we should not create a border item.
+  // It won't be rendered. Calling HasBorder() for themed frames is expensive
+  // too (calls into native theme code), so avoiding it is valuable.
+  if ((!bg || !bg->IsThemed()) && HasBorder()) {
     rv = aLists.BorderBackground()->AppendNewToTop(new (aBuilder)
         nsDisplayBorder(aBuilder, this));
     NS_ENSURE_SUCCESS(rv, rv);

layout/generic/nsFrame.h

@@ -130,6 +130,7 @@
 //----------------------------------------------------------------------
 
 struct nsBoxLayoutMetrics;
+class nsDisplayBackground;
 
 /**
  * Implementation of a simple frame that's not splittable and has no
@@ -509,10 +510,13 @@ public:
    * background style appears to have no background --- this is useful
    * for frames that might receive a propagated background via
    * nsCSSRendering::FindBackground
+   * @param aBackground *aBackground is set to the new nsDisplayBackground item,
+   * if one is created, otherwise null.
    */
   nsresult DisplayBackgroundUnconditional(nsDisplayListBuilder*   aBuilder,
                                           const nsDisplayListSet& aLists,
-                                          bool aForceBackground = false);
+                                          bool aForceBackground,
+                                          nsDisplayBackground** aBackground);
   /**
    * Adds display items for standard CSS borders, background and outline for
    * for this frame, as necessary. Checks IsVisibleForPainting and won't

layout/tables/nsTableFrame.cpp

@@ -1281,7 +1281,8 @@ nsTableFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
       // in its own display item, so do that to take advantage of
       // opacity and visibility optimizations
       if (deflate == nsMargin(0, 0, 0, 0)) {
-        nsresult rv = DisplayBackgroundUnconditional(aBuilder, aLists, false);
+        nsDisplayBackground* bg;
+        nsresult rv = DisplayBackgroundUnconditional(aBuilder, aLists, false, &bg);
         NS_ENSURE_SUCCESS(rv, rv);
       }
     }