diff --git a/mozilla/widget/public/nsILookAndFeel.h b/mozilla/widget/public/nsILookAndFeel.h
index 0c67550809c..1f33b258f69 100644
--- a/mozilla/widget/public/nsILookAndFeel.h
+++ b/mozilla/widget/public/nsILookAndFeel.h
@@ -110,7 +110,8 @@ public:
 
     eMetric_CaretBlinkTime,                               // default, may be overriden by OS
     eMetric_CaretWidthTwips,
-    eMetric_SubmenuDelay
+    eMetric_SubmenuDelay,
+    eMetric_MenusCanOverlapOSBar                          // can popups overlap menu/task bar?
   } nsMetricID;
 
   typedef enum {
diff --git a/mozilla/widget/src/gtk/nsLookAndFeel.cpp b/mozilla/widget/src/gtk/nsLookAndFeel.cpp
index 304e5adca53..453105341bb 100644
--- a/mozilla/widget/src/gtk/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/gtk/nsLookAndFeel.cpp
@@ -264,6 +264,10 @@ NS_IMETHODIMP nsLookAndFeel::GetMetric(const nsMetricID aID, PRInt32 & aMetric)
   case eMetric_SubmenuDelay:
     aMetric = 200;
     break;
+  case eMetric_MenusCanOverlapOSBar:
+    // we want XUL popups to be able to overlap the task bar.
+    aMetric = 1;
+    break;
   default:
     aMetric = 0;
     res     = NS_ERROR_FAILURE;
diff --git a/mozilla/widget/src/mac/nsLookAndFeel.cpp b/mozilla/widget/src/mac/nsLookAndFeel.cpp
index 44824c2015b..ee37aa244f7 100644
--- a/mozilla/widget/src/mac/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/mac/nsLookAndFeel.cpp
@@ -250,6 +250,10 @@ NS_IMETHODIMP nsLookAndFeel::GetMetric(const nsMetricID aID, PRInt32 & aMetric)
     case eMetric_SubmenuDelay:
         aMetric = 200;
         break;
+    case eMetric_MenusCanOverlapOSBar:
+        // xul popups are not allowed to overlap the menubar.
+        aMetric = 0;
+        break;
     default:
         aMetric = 0;
         res = NS_ERROR_FAILURE;
diff --git a/mozilla/widget/src/windows/nsLookAndFeel.cpp b/mozilla/widget/src/windows/nsLookAndFeel.cpp
index 485d4a48c36..759463d1688 100644
--- a/mozilla/widget/src/windows/nsLookAndFeel.cpp
+++ b/mozilla/widget/src/windows/nsLookAndFeel.cpp
@@ -275,6 +275,11 @@ NS_IMETHODIMP nsLookAndFeel::GetMetric(const nsMetricID aID, PRInt32 & aMetric)
         aMetric = sSubmenuDelay;
         }
         break;
+    case eMetric_MenusCanOverlapOSBar:
+        // we want XUL popups to be able to overlap the task bar.
+        aMetric = 1;
+        break;
+
     default:
         aMetric = -1;
         res = NS_ERROR_FAILURE;