diff --git a/layout/base/nsStyleConsts.h b/layout/base/nsStyleConsts.h
index 17be3a91a9cf..72d924195210 100644
--- a/layout/base/nsStyleConsts.h
+++ b/layout/base/nsStyleConsts.h
@@ -42,6 +42,7 @@
 #define nsStyleConsts_h___
 
 #include "nsFont.h"
+#include "nsIWidget.h"
 
 // cairo doesn't support invert
 // #define GFX_HAS_INVERT
@@ -710,10 +711,6 @@
 #define NS_STYLE_IME_MODE_DISABLED              3
 #define NS_STYLE_IME_MODE_INACTIVE              4
 
-// See nsStyleUIReset
-#define NS_STYLE_WINDOW_SHADOW_NONE             0
-#define NS_STYLE_WINDOW_SHADOW_DEFAULT          1
-
 #ifdef MOZ_SVG
 // See nsStyleSVG
 
diff --git a/layout/generic/nsContainerFrame.cpp b/layout/generic/nsContainerFrame.cpp
index b7e744745c33..9c03d22c0785 100644
--- a/layout/generic/nsContainerFrame.cpp
+++ b/layout/generic/nsContainerFrame.cpp
@@ -508,7 +508,10 @@ SyncFrameViewGeometryDependentProperties(nsPresContext*  aPresContext,
       nsIFrame *rootFrame = aPresContext->PresShell()->FrameConstructor()->GetRootElementStyleFrame();
       if(rootFrame && NS_THEME_WIN_GLASS == rootFrame->GetStyleDisplay()->mAppearance)
         mode = eTransparencyGlass;
-      aView->GetWidget()->SetTransparencyMode(mode);
+      nsIWidget* widget = aView->GetWidget();
+      widget->SetTransparencyMode(mode);
+      if (rootFrame)
+        widget->SetWindowShadowStyle(rootFrame->GetStyleUIReset()->mWindowShadow);
     }
   }
 }
diff --git a/layout/xul/base/src/nsMenuPopupFrame.cpp b/layout/xul/base/src/nsMenuPopupFrame.cpp
index c1ccf5a63df9..2105772b268a 100644
--- a/layout/xul/base/src/nsMenuPopupFrame.cpp
+++ b/layout/xul/base/src/nsMenuPopupFrame.cpp
@@ -280,7 +280,10 @@ nsMenuPopupFrame::CreateWidgetForView(nsIView* aView)
   aView->CreateWidget(kCChildCID, &widgetData, nsnull, PR_TRUE, PR_TRUE,
                       eContentTypeInherit, parentWidget);
 #endif
-  aView->GetWidget()->SetTransparencyMode(mode);
+  nsIWidget* widget = aView->GetWidget();
+  widget->SetTransparencyMode(viewHasTransparentContent ?
+      eTransparencyTransparent : eTransparencyOpaque);
+  widget->SetWindowShadowStyle(GetStyleUIReset()->mWindowShadow);
   return NS_OK;
 }
 
diff --git a/widget/public/nsIWidget.h b/widget/public/nsIWidget.h
index 2271da83f790..7296b70ada6f 100644
--- a/widget/public/nsIWidget.h
+++ b/widget/public/nsIWidget.h
@@ -93,16 +93,24 @@ typedef nsEventStatus (* EVENT_CALLBACK)(nsGUIEvent *event);
 #define NS_NATIVE_PLUGIN_PORT_CG    101
 #endif
 
-// 0e64821f-00a2-4adc-ac3b-3439d61f4491
+// 8c91457a-ef86-4da1-b4f9-36022dcc6c7e
 #define NS_IWIDGET_IID \
-{ 0x0e64821f, 0x00a2, 0x4adc, \
-  { 0xac, 0x3b, 0x34, 0x39, 0xd6, 0x1f, 0x44, 0x91 } }
+{ 0x8c91457a, 0xef86, 0x4da1, \
+  { 0xb4, 0xf9, 0x36, 0x02, 0x2d, 0xcc, 0x6c, 0x7e } }
 
 // Hide the native window systems real window type so as to avoid
 // including native window system types and APIs. This is necessary
 // to ensure cross-platform code.
 typedef void* nsNativeWidget;
 
+/*
+ * Window shadow styles
+ * Also used for the -moz-window-shadow CSS property
+ */
+
+#define NS_STYLE_WINDOW_SHADOW_NONE             0
+#define NS_STYLE_WINDOW_SHADOW_DEFAULT          1
+
 /**
  * Border styles
  */
@@ -711,6 +719,11 @@ class nsIWidget : public nsISupports {
      */
     virtual nsTransparencyMode GetTransparencyMode() = 0;
 
+    /**
+     * Set the shadow style of the window.
+     */
+    NS_IMETHOD SetWindowShadowStyle(PRInt32 aStyle) = 0;
+
     /** 
      * Hide window chrome (borders, buttons) for this widget.
      *
diff --git a/widget/src/cocoa/nsChildView.h b/widget/src/cocoa/nsChildView.h
index c11a07223785..37375329c489 100644
--- a/widget/src/cocoa/nsChildView.h
+++ b/widget/src/cocoa/nsChildView.h
@@ -166,6 +166,8 @@ enum {
   // All views are always opaque (non-transparent). The only exception is when we're
   // the content view in a transparent XUL window.
   BOOL mIsTransparent;
+  PRIntervalTime mLastShadowInvalidation;
+  BOOL mNeedsShadowInvalidation;
 
   // Holds our drag service across multiple drag calls. The reference to the
   // service is obtained when the mouse enters the view and is released when
@@ -366,6 +368,7 @@ public:
   
   virtual nsTransparencyMode GetTransparencyMode();
   virtual void                SetTransparencyMode(nsTransparencyMode aMode);
+  NS_IMETHOD        SetWindowShadowStyle(PRInt32 aStyle);
   
   // Mac specific methods
   virtual PRBool    PointInWidget(Point aThePoint);
diff --git a/widget/src/cocoa/nsChildView.mm b/widget/src/cocoa/nsChildView.mm
index aa8722af3de4..24d57d7dfe18 100644
--- a/widget/src/cocoa/nsChildView.mm
+++ b/widget/src/cocoa/nsChildView.mm
@@ -179,6 +179,9 @@ nsIWidget         * gRollupWidget   = nsnull;
 
 - (BOOL)isPaintingSuppressed;
 
+- (void)maybeInvalidateShadow;
+- (void)invalidateShadow;
+
 #if USE_CLICK_HOLD_CONTEXTMENU
  // called on a timer two seconds after a mouse down to see if we should display
  // a context menu (click-hold)
@@ -777,6 +780,28 @@ void nsChildView::SetTransparencyMode(nsTransparencyMode aMode)
 }
 
 
+// This is called by nsContainerFrame on the root widget for all window types
+// except popup windows (when nsCocoaWindow::SetWindowShadowStyle is used instead).
+NS_IMETHODIMP nsChildView::SetWindowShadowStyle(PRInt32 aStyle)
+{
+  NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+  // Find out if this is a window we created by seeing if the delegate is WindowDelegate. If it is,
+  // tell the nsCocoaWindow to set the shadow style.
+  id windowDelegate = [[mView nativeWindow] delegate];
+  if (windowDelegate && [windowDelegate isKindOfClass:[WindowDelegate class]]) {
+    nsCocoaWindow *widget = [(WindowDelegate *)windowDelegate geckoWidget];
+    if (widget) {
+      widget->SetWindowShadowStyle(aStyle);
+    }
+  }
+
+  return NS_OK;
+
+  NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}
+
+
 NS_IMETHODIMP nsChildView::IsVisible(PRBool& outState)
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
@@ -2733,6 +2758,38 @@ NSEvent* gLastDragEvent = nil;
 }
 
 
+// Limit shadow invalidation to 10 times per second.
+static const PRInt32 sShadowInvalidationInterval = 100;
+- (void)maybeInvalidateShadow
+{
+  if (!mIsTransparent || ![mWindow hasShadow])
+    return;
+
+  PRIntervalTime now = PR_IntervalNow();
+  PRInt32 elapsed = PR_IntervalToMilliseconds(now - mLastShadowInvalidation);
+  if (!mLastShadowInvalidation ||
+      elapsed >= sShadowInvalidationInterval) {
+    [mWindow invalidateShadow];
+    mLastShadowInvalidation = now;
+    mNeedsShadowInvalidation = NO;
+  } else if (!mNeedsShadowInvalidation) {
+    mNeedsShadowInvalidation = YES;
+    [self performSelector:@selector(invalidateShadow)
+               withObject:nil
+               afterDelay:(sShadowInvalidationInterval - elapsed) / 1000.0];
+  }
+}
+
+
+- (void)invalidateShadow
+{
+  if (!mNeedsShadowInvalidation)
+    return;
+  [mWindow invalidateShadow];
+  mNeedsShadowInvalidation = NO;
+}
+
+
 - (BOOL)isPaintingSuppressed
 {
   NSWindow* win = [self window];
@@ -2811,6 +2868,10 @@ NSEvent* gLastDragEvent = nil;
   if (!mGeckoChild)
     return;
 
+  // If we're a transparent window, and our contents have changed, we need
+  // to make sure the shadow is updated to the new contents.
+  [self maybeInvalidateShadow];
+
   paintEvent.renderingContext = nsnull;
   paintEvent.region = nsnull;
 
diff --git a/widget/src/cocoa/nsCocoaWindow.h b/widget/src/cocoa/nsCocoaWindow.h
index 8c59903d7b36..b549341a61e3 100644
--- a/widget/src/cocoa/nsCocoaWindow.h
+++ b/widget/src/cocoa/nsCocoaWindow.h
@@ -276,6 +276,7 @@ public:
     NS_IMETHOD GetAttention(PRInt32 aCycleCount);
     virtual nsTransparencyMode GetTransparencyMode();
     virtual void SetTransparencyMode(nsTransparencyMode aMode);
+    NS_IMETHOD SetWindowShadowStyle(PRInt32 aStyle);
     NS_IMETHOD SetWindowTitlebarColor(nscolor aColor, PRBool aActive);
 
     // dispatch an NS_SIZEMODE event on miniaturize or deminiaturize
diff --git a/widget/src/cocoa/nsCocoaWindow.mm b/widget/src/cocoa/nsCocoaWindow.mm
index c6fe916558d0..8f570ab431e9 100644
--- a/widget/src/cocoa/nsCocoaWindow.mm
+++ b/widget/src/cocoa/nsCocoaWindow.mm
@@ -65,6 +65,7 @@
 #include "nsIDOMElement.h"
 #include "nsMenuBarX.h"
 #include "nsMenuUtilsX.h"
+#include "nsStyleConsts.h"
 
 #include "gfxPlatform.h"
 #include "lcms.h"
@@ -800,7 +801,6 @@ void nsCocoaWindow::MakeBackgroundTransparent(PRBool aTransparent)
     }
     [mWindow setOpaque:!aTransparent];
     [mWindow setBackgroundColor:(aTransparent ? [NSColor clearColor] : [NSColor whiteColor])];
-    [mWindow setHasShadow:!aTransparent];
   }
 
   NS_OBJC_END_TRY_ABORT_BLOCK;
@@ -1317,6 +1317,18 @@ NS_IMETHODIMP nsCocoaWindow::GetAttention(PRInt32 aCycleCount)
 }
 
 
+NS_IMETHODIMP nsCocoaWindow::SetWindowShadowStyle(PRInt32 aStyle)
+{
+  NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
+
+  if ([mWindow hasShadow] != (aStyle != NS_STYLE_WINDOW_SHADOW_NONE))
+    [mWindow setHasShadow:(aStyle != NS_STYLE_WINDOW_SHADOW_NONE)];
+  return NS_OK;
+
+  NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
+}
+
+
 NS_IMETHODIMP nsCocoaWindow::SetWindowTitlebarColor(nscolor aColor, PRBool aActive)
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
diff --git a/widget/src/xpwidgets/nsBaseWidget.cpp b/widget/src/xpwidgets/nsBaseWidget.cpp
index 01a46159cb97..576233a4c9d2 100644
--- a/widget/src/xpwidgets/nsBaseWidget.cpp
+++ b/widget/src/xpwidgets/nsBaseWidget.cpp
@@ -568,6 +568,17 @@ nsTransparencyMode nsBaseWidget::GetTransparencyMode() {
   return eTransparencyOpaque;
 }
 
+//-------------------------------------------------------------------------
+//
+// Set window shadow style
+//
+//-------------------------------------------------------------------------
+
+NS_IMETHODIMP nsBaseWidget::SetWindowShadowStyle(PRInt32 aMode)
+{
+  return NS_ERROR_NOT_IMPLEMENTED;
+}
+
 //-------------------------------------------------------------------------
 //
 // Hide window borders/decorations for this widget
diff --git a/widget/src/xpwidgets/nsBaseWidget.h b/widget/src/xpwidgets/nsBaseWidget.h
index e15d1b9fd55c..2770d1dcdb70 100644
--- a/widget/src/xpwidgets/nsBaseWidget.h
+++ b/widget/src/xpwidgets/nsBaseWidget.h
@@ -106,6 +106,7 @@ public:
   NS_IMETHOD              SetWindowType(nsWindowType aWindowType);
   virtual void            SetTransparencyMode(nsTransparencyMode aMode);
   virtual nsTransparencyMode GetTransparencyMode();
+  NS_IMETHOD              SetWindowShadowStyle(PRInt32 aStyle);
   NS_IMETHOD              HideWindowChrome(PRBool aShouldHide);
   NS_IMETHOD              MakeFullScreen(PRBool aFullScreen);
   virtual nsIRenderingContext* GetRenderingContext();