From 662a18f196b0d14585f0f4f41ccc1128210462db Mon Sep 17 00:00:00 2001
From: Mats Palmgren <mats@mozilla.com>
Date: Tue, 6 Jan 2015 09:27:56 +0000
Subject: [PATCH] Bug 1116718 part 1 - Don't bother calculating overflow areas
 when they will be discarded anyway.  r=roc

---
 layout/generic/nsViewportFrame.cpp | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/layout/generic/nsViewportFrame.cpp b/layout/generic/nsViewportFrame.cpp
index 0f51337414b..b140e14f392 100644
--- a/layout/generic/nsViewportFrame.cpp
+++ b/layout/generic/nsViewportFrame.cpp
@@ -252,18 +252,16 @@ ViewportFrame::Reflow(nsPresContext*           aPresContext,
     }
 
     nsRect rect = AdjustReflowStateAsContainingBlock(&reflowState);
-
-    // Just reflow all the fixed-pos frames.
-    GetAbsoluteContainingBlock()->Reflow(this, aPresContext, reflowState, aStatus,
-                                         rect,
-                                         false, true, true, // XXX could be optimized
-                                         &aDesiredSize.mOverflowAreas);
-
+    nsOverflowAreas* overflowAreas = &aDesiredSize.mOverflowAreas;
     nsIScrollableFrame* rootScrollFrame =
                     aPresContext->PresShell()->GetRootScrollFrameAsScrollable();
     if (rootScrollFrame && !rootScrollFrame->IsIgnoringViewportClipping()) {
-      aDesiredSize.SetOverflowAreasToDesiredBounds();
+      overflowAreas = nullptr;
     }
+    GetAbsoluteContainingBlock()->Reflow(this, aPresContext, reflowState, aStatus,
+                                         rect,
+                                         false, true, true, // XXX could be optimized
+                                         overflowAreas);
   }
 
   if (mFrames.NotEmpty()) {