diff --git a/gfx/layers/basic/BasicLayerManager.cpp b/gfx/layers/basic/BasicLayerManager.cpp
index 3d6f28050bc3..2388af5a8116 100644
--- a/gfx/layers/basic/BasicLayerManager.cpp
+++ b/gfx/layers/basic/BasicLayerManager.cpp
@@ -21,6 +21,7 @@
 #include "BasicLayersImpl.h"
 #include "BasicThebesLayer.h"
 #include "BasicContainerLayer.h"
+#include "mozilla/Preferences.h"
 
 using namespace mozilla::gfx;
 
@@ -434,6 +435,9 @@ BasicLayerManager::EndTransactionInternal(DrawThebesLayerCallback aCallback,
     }
 
     PaintLayer(mTarget, mRoot, aCallback, aCallbackData, nsnull);
+    if (mWidget) {
+      FlashWidgetUpdateArea(mTarget);
+    }
 
     if (!mTransactionIncomplete) {
       // Clear out target if we have a complete transaction.
@@ -466,6 +470,27 @@ BasicLayerManager::EndTransactionInternal(DrawThebesLayerCallback aCallback,
   return !mTransactionIncomplete;
 }
 
+void
+BasicLayerManager::FlashWidgetUpdateArea(gfxContext *aContext)
+{
+  static bool sWidgetFlashingEnabled;
+  static bool sWidgetFlashingPrefCached = false;
+
+  if (!sWidgetFlashingPrefCached) {
+    sWidgetFlashingPrefCached = true;
+    mozilla::Preferences::AddBoolVarCache(&sWidgetFlashingEnabled,
+                                          "nglayout.debug.widget_update_flashing");
+  }
+
+  if (sWidgetFlashingEnabled) {
+    float r = float(rand()) / RAND_MAX;
+    float g = float(rand()) / RAND_MAX;
+    float b = float(rand()) / RAND_MAX;
+    aContext->SetColor(gfxRGBA(r, g, b, 0.2));
+    aContext->Paint();
+  }
+}
+
 bool
 BasicLayerManager::EndEmptyTransaction()
 {
diff --git a/gfx/layers/basic/BasicLayers.h b/gfx/layers/basic/BasicLayers.h
index b8d4e0b38777..ed64021b3678 100644
--- a/gfx/layers/basic/BasicLayers.h
+++ b/gfx/layers/basic/BasicLayers.h
@@ -172,6 +172,8 @@ protected:
                               void* aCallbackData,
                               EndTransactionFlags aFlags = END_DEFAULT);
 
+  void FlashWidgetUpdateArea(gfxContext* aContext);
+
   // Widget whose surface should be used as the basis for ThebesLayer
   // buffers.
   nsIWidget* mWidget;
diff --git a/modules/libpref/src/init/all.js b/modules/libpref/src/init/all.js
index b8dc82e27100..5dc1f23bbd0a 100644
--- a/modules/libpref/src/init/all.js
+++ b/modules/libpref/src/init/all.js
@@ -353,6 +353,10 @@ pref("nglayout.enable_drag_images", true);
 // enable/disable paint flashing --- useful for debugging
 pref("nglayout.debug.paint_flashing", false);
 
+// enable/disable widget update area flashing --- only supported with 
+// BasicLayers (other layer managers always update the entire widget area)
+pref("nglayout.debug.widget_update_flashing", false);
+
 // scrollbar snapping region
 // 0 - off
 // 1 and higher - slider thickness multiple