Bug 959847. Part 7. Move ignoreViewportScrolling computation lower, where we actually use it. r=roc

layout/base/nsLayoutUtils.cpp

@@ -2226,8 +2226,6 @@ nsLayoutUtils::PaintFrame(nsRenderingContext* aRenderingContext, nsIFrame* aFram
     }
   }
 
-  bool ignoreViewportScrolling =
-    aFrame->GetParent() ? false : presShell->IgnoringViewportScrolling();
   nsRegion visibleRegion;
   if (aFlags & PAINT_WIDGET_LAYERS) {
     // This layer tree will be reused, so we'll need to calculate it
@@ -2275,6 +2273,8 @@ nsLayoutUtils::PaintFrame(nsRenderingContext* aRenderingContext, nsIFrame* aFram
   }
   nsRect canvasArea(nsPoint(0, 0), aFrame->GetSize());
 
+  bool ignoreViewportScrolling =
+    aFrame->GetParent() ? false : presShell->IgnoringViewportScrolling();
   if (ignoreViewportScrolling && rootScrollFrame) {
     nsIScrollableFrame* rootScrollableFrame =
       presShell->GetRootScrollFrameAsScrollable();