Bug 961431. Round the final screen position of the popup to pixels, not the offset between it's view and parent view. r=enndeakin

layout/xul/nsMenuPopupFrame.cpp

@@ -1334,14 +1334,15 @@ nsMenuPopupFrame::SetPopupPosition(nsIFrame* aAnchorFrame, bool aIsMove, bool aS
                  "Popup is offscreen");
   }
 
+  // snap the popup's position in screen coordinates to device pixels,
+  // see bug 622507, bug 961431
+  screenPoint.x = presContext->RoundAppUnitsToNearestDevPixels(screenPoint.x);
+  screenPoint.y = presContext->RoundAppUnitsToNearestDevPixels(screenPoint.y);
+
   // determine the x and y position of the view by subtracting the desired
   // screen position from the screen position of the root frame.
   nsPoint viewPoint = screenPoint - rootScreenRect.TopLeft();
 
-  // snap the view's position to device pixels, see bug 622507
-  viewPoint.x = presContext->RoundAppUnitsToNearestDevPixels(viewPoint.x);
-  viewPoint.y = presContext->RoundAppUnitsToNearestDevPixels(viewPoint.y);
-
   nsView* view = GetView();
   NS_ASSERTION(view, "popup with no view");