Bug 1379344 - Avoid traversing the display list twice inside nsDisplayList::UpdateBounds(); r=mstange

layout/painting/nsDisplayList.cpp

@@ -1892,7 +1892,8 @@ nsDisplayList::GetBounds(nsDisplayListBuilder* aBuilder) const {
 
 nsRect
 nsDisplayList::GetClippedBoundsWithRespectToASR(nsDisplayListBuilder* aBuilder,
-                                                const ActiveScrolledRoot* aASR) const {
+                                                const ActiveScrolledRoot* aASR,
+                                                nsRect* aVisibleRect) const {
   nsRect bounds;
   for (nsDisplayItem* i = GetBottom(); i != nullptr; i = i->GetAbove()) {
     nsRect r = i->GetClippedBounds(aBuilder);
@@ -1908,6 +1909,9 @@ nsDisplayList::GetClippedBoundsWithRespectToASR(nsDisplayListBuilder* aBuilder,
         r = clip->GetClipRect();
       }
     }
+    if (aVisibleRect) {
+      aVisibleRect->UnionRect(*aVisibleRect, i->GetVisibleRect());
+    }
     bounds.UnionRect(bounds, r);
   }
   return bounds;

layout/painting/nsDisplayList.h

@@ -2517,9 +2517,14 @@ public:
    * If there is an item in this list which is not bounded with respect to
    * aASR (i.e. which does not have "finite bounds" with respect to aASR),
    * then this method trigger an assertion failure.
+   * The optional aVisibleRect out argument can be set to non-null if the
+   * caller is also interested to know the visible rect.  This can be used
+   * to get the visible rect efficiently without traversing the display list
+   * twice.
    */
   nsRect GetClippedBoundsWithRespectToASR(nsDisplayListBuilder* aBuilder,
-                                          const ActiveScrolledRoot* aASR) const;
+                                          const ActiveScrolledRoot* aASR,
+                                          nsRect* aVisibleRect = nullptr) const;
 
   /**
    * Find the topmost display item that returns a non-null frame, and return
@@ -3822,7 +3827,9 @@ public:
    */
   virtual void UpdateBounds(nsDisplayListBuilder* aBuilder) override
   {
-    mBounds = mList.GetClippedBoundsWithRespectToASR(aBuilder, mActiveScrolledRoot);
+    nsRect visibleRect;
+    mBounds = mList.GetClippedBoundsWithRespectToASR(aBuilder, mActiveScrolledRoot,
+                                                     &visibleRect);
     // The display list may contain content that's visible outside the visible
     // rect (i.e. the current dirty rect) passed in when the item was created.
     // This happens when the dirty rect has been restricted to the visual
@@ -3830,7 +3837,7 @@ public:
     // rects in nsDisplayListBuilder::MarkOutOfFlowFrameForDisplay), but that
     // frame contains placeholders for out-of-flows that aren't descendants of
     // the frame.
-    mVisibleRect.UnionRect(mBaseVisibleRect, mList.GetVisibleRect());
+    mVisibleRect.UnionRect(mBaseVisibleRect, visibleRect);
   }
   virtual void HitTest(nsDisplayListBuilder* aBuilder, const nsRect& aRect,
                        HitTestState* aState, nsTArray<nsIFrame*> *aOutFrames) override;