diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 97f457382f08..94891595c4a1 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -3036,31 +3036,34 @@ nsLayoutUtils::HasNonZeroCornerOnSide(const nsStyleCorners& aCorners,
 }
 
 /* static */ nsTransparencyMode
-nsLayoutUtils::GetFrameTransparency(nsIFrame* aFrame) {
-  if (aFrame->GetStyleContext()->GetStyleDisplay()->mOpacity < 1.0f)
+nsLayoutUtils::GetFrameTransparency(nsIFrame* aBackgroundFrame,
+                                    nsIFrame* aCSSRootFrame) {
+  if (aCSSRootFrame->GetStyleContext()->GetStyleDisplay()->mOpacity < 1.0f)
     return eTransparencyTransparent;
 
-  if (HasNonZeroCorner(aFrame->GetStyleContext()->GetStyleBorder()->mBorderRadius))
+  if (HasNonZeroCorner(aCSSRootFrame->GetStyleContext()->GetStyleBorder()->mBorderRadius))
     return eTransparencyTransparent;
 
   nsTransparencyMode transparency;
-  if (aFrame->IsThemed(&transparency))
+  if (aCSSRootFrame->IsThemed(&transparency))
     return transparency;
 
-  if (aFrame->GetStyleDisplay()->mAppearance == NS_THEME_WIN_GLASS)
+  if (aCSSRootFrame->GetStyleDisplay()->mAppearance == NS_THEME_WIN_GLASS)
     return eTransparencyGlass;
 
   // We need an uninitialized window to be treated as opaque because
   // doing otherwise breaks window display effects on some platforms,
   // specifically Vista. (bug 450322)
-  if (aFrame->GetType() == nsGkAtoms::viewportFrame &&
-      !aFrame->GetFirstChild(nsnull)) {
+  if (aBackgroundFrame->GetType() == nsGkAtoms::viewportFrame &&
+      !aBackgroundFrame->GetFirstChild(nsnull)) {
     return eTransparencyOpaque;
   }
 
   const nsStyleBackground* bg;
-  if (!nsCSSRendering::FindBackground(aFrame->PresContext(), aFrame, &bg))
+  if (!nsCSSRendering::FindBackground(aBackgroundFrame->PresContext(),
+                                      aBackgroundFrame, &bg)) {
     return eTransparencyTransparent;
+  }
   if (NS_GET_A(bg->mBackgroundColor) < 255 ||
       // bottom layer's clip is used for the color
       bg->BottomLayer().mClip != NS_STYLE_BG_CLIP_BORDER)
diff --git a/layout/base/nsLayoutUtils.h b/layout/base/nsLayoutUtils.h
index b270c80adb56..736a1cfe3b06 100644
--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -961,10 +961,16 @@ public:
 
   /**
    * Determine if a widget is likely to require transparency or translucency.
-   *   @param aFrame the frame of a <window>, <popup> or <menupopup> element.
+   *   @param aBackgroundFrame The frame that the background is set on. For
+   *                           <window>s, this will be the canvas frame.
+   *   @param aCSSRootFrame    The frame that holds CSS properties affecting
+   *                           the widget's transparency. For menupopups,
+   *                           aBackgroundFrame and aCSSRootFrame will be the
+   *                           same.
    *   @return a value suitable for passing to SetWindowTranslucency
    */
-  static nsTransparencyMode GetFrameTransparency(nsIFrame* aFrame);
+  static nsTransparencyMode GetFrameTransparency(nsIFrame* aBackgroundFrame,
+                                                 nsIFrame* aCSSRootFrame);
 
   /**
    * Get textrun construction flags determined by a given style; in particular
diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index e281b52bd31e..4d9ba2e8c151 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -539,14 +539,7 @@ nsContainerFrame::SyncWindowProperties(nsPresContext*       aPresContext,
   if (!rootFrame)
     return;
 
-  nsTransparencyMode mode = nsLayoutUtils::GetFrameTransparency(aFrame);
-  // The issue here is that the CSS 'background' propagates from the root
-  // element's frame (rootFrame) to the real root frame (nsViewportFrame),
-  // so we need to call GetFrameTransparency on that. But -moz-appearance
-  // does not propagate so we need to check that directly on rootFrame.
-  if (NS_THEME_WIN_GLASS == rootFrame->GetStyleDisplay()->mAppearance) {
-    mode = eTransparencyGlass;
-  }
+  nsTransparencyMode mode = nsLayoutUtils::GetFrameTransparency(aFrame, rootFrame);
   nsIWidget* viewWidget = aView->GetWidget();
   viewWidget->SetTransparencyMode(mode);
   windowWidget->SetWindowShadowStyle(rootFrame->GetStyleUIReset()->mWindowShadow);
diff --git a/layout/xul/base/src/nsMenuPopupFrame.cpp b/layout/xul/base/src/nsMenuPopupFrame.cpp
index 8853c97a6761..f94496598d8d 100644
--- a/layout/xul/base/src/nsMenuPopupFrame.cpp
+++ b/layout/xul/base/src/nsMenuPopupFrame.cpp
@@ -251,7 +251,7 @@ nsMenuPopupFrame::CreateWidgetForView(nsIView* aView)
   widgetData.clipSiblings = PR_TRUE;
   widgetData.mPopupHint = mPopupType;
 
-  nsTransparencyMode mode = nsLayoutUtils::GetFrameTransparency(this);
+  nsTransparencyMode mode = nsLayoutUtils::GetFrameTransparency(this, this);
   PRBool viewHasTransparentContent = !mInContentShell &&
                                      (eTransparencyTransparent ==
                                       mode);