Bug 350148. Improve scrolling of large documents generally by only searching descendants of the moving frame for position-varying graphics when we know we've actually painted position-varying graphics in this prescontext. r+sr=dbaron

layout/base/nsCSSRendering.cpp

@@ -3092,6 +3092,9 @@ nsCSSRendering::PaintBackgroundWithSC(nsPresContext* aPresContext,
     nsIView* viewportView = nsnull;
     nsRect viewportArea;
 
+    // Remember that we've drawn position-varying content in this prescontext
+    aPresContext->SetRenderedPositionVaryingContent();
+
     nsIFrame* rootFrame =
       aPresContext->PresShell()->FrameManager()->GetRootFrame();
     NS_ASSERTION(rootFrame, "no root frame");

layout/base/nsLayoutUtils.cpp

@@ -793,6 +793,9 @@ nsLayoutUtils::ComputeRepaintRegionForCopy(nsIFrame* aRootFrame,
                                            const nsRect& aCopyRect,
                                            nsRegion* aRepaintRegion)
 {
+  NS_ASSERTION(aRootFrame != aMovingFrame,
+               "The root frame shouldn't be the one that's moving, that makes no sense");
+
   // Build the 'after' display list over the whole area of interest.
   // Frames under aMovingFrame will not be allowed to affect (clip or cover)
   // non-moving frame display items ... then we can be sure the non-moving

layout/base/nsPresContext.h

@@ -387,6 +387,9 @@ public:
    */
   PRBool IsPaginated() const { return mPaginated; }
   
+  PRBool GetRenderedPositionVaryingContent() const { return mRenderedPositionVaryingContent; }
+  void SetRenderedPositionVaryingContent() { mRenderedPositionVaryingContent = PR_TRUE; }
+
   /**
    * Sets whether the presentation context can scroll for a paginated
    * context.
@@ -762,6 +765,7 @@ protected:
   unsigned              mPrefScrollbarSide : 2;
   unsigned              mPendingSysColorChanged : 1;
   unsigned              mPendingThemeChanged : 1;
+  unsigned              mRenderedPositionVaryingContent : 1;
 
 #ifdef IBMBIDI
   unsigned              mIsVisual : 1;

layout/generic/nsFrame.cpp

@@ -1155,8 +1155,6 @@ nsIFrame::BuildDisplayListForStackingContext(nsDisplayListBuilder* aBuilder,
   if ((GetStateBits() & NS_FRAME_REPLACED_ELEMENT) &&
       !IsVisibleForPainting(aBuilder))
     return NS_OK;
-  if (GetStyleVisibility()->mVisible == NS_STYLE_VISIBILITY_COLLAPSE)
-    return NS_OK;
 
   nsRect absPosClip;
   const nsStyleDisplay* disp = GetStyleDisplay();
@@ -1337,19 +1335,29 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
     dirty.IntersectRect(dirty, aChild->GetOverflowRect());
   }
 
-  // If this child has a placeholder of interest then we must descend into
-  // it even if the child's descendant frames don't intersect the dirty
-  // area themselves.
+  if (!(aChild->GetStateBits() & NS_FRAME_FORCE_DISPLAY_LIST_DESCEND_INTO)) {
+    // No need to descend into aChild to catch placeholders for visible
+    // positioned stuff. So see if we can short-circuit frame traversal here.
+
+    // We can stop if aChild's intersection with the dirty area ended up empty.
     // If the child is a scrollframe that we want to ignore, then we need
     // to descend into it because its scrolled child may intersect the dirty
     // area even if the scrollframe itself doesn't.
-  if (dirty.IsEmpty() &&
-      !(aChild->GetStateBits() & NS_FRAME_FORCE_DISPLAY_LIST_DESCEND_INTO) && 
-      aChild != aBuilder->GetIgnoreScrollFrame())
+    if (dirty.IsEmpty() && aChild != aBuilder->GetIgnoreScrollFrame())
       return NS_OK;
 
-  if (aChild->GetStyleVisibility()->mVisible == NS_STYLE_VISIBILITY_COLLAPSE)
+    // Note that aBuilder->GetRootMovingFrame() is non-null only if we're doing
+    // ComputeRepaintRegionForCopy.
+    if (aBuilder->GetRootMovingFrame() == this &&
+        !GetPresContext()->GetRenderedPositionVaryingContent()) {
+      // No position-varying content has been rendered in this prescontext.
+      // Therefore there is no need to descend into analyzing the moving frame's
+      // descendants looking for such content, because any bitblit will
+      // not be copying position-varying graphics.
       return NS_OK;
+    }
+  }
+
   // XXX need to have inline-block and inline-table set pseudoStackingContext
   
   const nsStyleDisplay* ourDisp = GetStyleDisplay();