109     , m_fixedObjectCount(0)

   761 }

   762 

   763 void FrameView::addFixedObject()

   764 {

   765     if (!m_fixedObjectCount && platformWidget())

   766         setCanBlitOnScroll(false);

   767     ++m_fixedObjectCount;

   768 }

   769 

   770 void FrameView::removeFixedObject()

   771 {

   772     ASSERT(m_fixedObjectCount > 0);

   773     m_fixedObjectCount--;

   774     if (!m_fixedObjectCount)

   775         setCanBlitOnScroll(!useSlowRepaints());

   776 }

   777 

   778 void FrameView::scrollContentsFastPath(const IntSize& scrollDelta, const IntRect& rectToScroll, const IntRect& clipRect)

   779 {

   780     const size_t fixedObjectThreshold = 5;

   781 

   782     ListHashSet<RenderBox*>* positionedObjects = 0;

   783     if (RenderView* root = m_frame->contentRenderer())

   784         positionedObjects = root->positionedObjects();

   785 

   786     if (!positionedObjects || positionedObjects->isEmpty()) {

   787         hostWindow()->scroll(scrollDelta, rectToScroll, clipRect);

   788         return;

   789     }

   790 

   791     // Get the rects of the fixed objects visible in the rectToScroll

   792     Vector<IntRect, fixedObjectThreshold> subRectToUpdate;

   793     bool updateInvalidatedSubRect = true;

   794     ListHashSet<RenderBox*>::const_iterator end = positionedObjects->end();

   795     for (ListHashSet<RenderBox*>::const_iterator it = positionedObjects->begin(); it != end; ++it) {

   796         RenderBox* renderBox = *it;

   797         if (renderBox->style()->position() != FixedPosition)

   798             continue;

   799         IntRect topLevelRect;

   800         IntRect updateRect = renderBox->paintingRootRect(topLevelRect);

   801         updateRect.move(-scrollX(), -scrollY());

   802         updateRect.intersect(rectToScroll);

   803         if (!updateRect.isEmpty()) {

   804             if (subRectToUpdate.size() >= fixedObjectThreshold) {

   805                 updateInvalidatedSubRect = false;

   806                 break;

   807             }

   808             subRectToUpdate.append(updateRect);

   809         }

   810     }

   811 

   812     // Scroll the view

   813     if (updateInvalidatedSubRect) {

   814         // 1) scroll

   815         hostWindow()->scroll(scrollDelta, rectToScroll, clipRect);

   816 

   817         // 2) update the area of fixed objets that has been invalidated

   818         size_t fixObjectsCount = subRectToUpdate.size();

   819         for (size_t i = 0; i < fixObjectsCount; ++i) {

   820             IntRect updateRect = subRectToUpdate[i];

   821             IntRect scrolledRect = updateRect;

   822             scrolledRect.move(scrollDelta);

   823             updateRect.unite(scrolledRect);

   824             updateRect.intersect(rectToScroll);

   825             hostWindow()->repaint(updateRect, true, false, true);

   826         }

   827     } else {

   828         // the number of fixed objects exceed the threshold, so we repaint everything.

   829         IntRect updateRect = clipRect;

   830         updateRect.intersect(rectToScroll);

   831         hostWindow()->repaint(updateRect, true, false, true);

   832     }