diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index 012a4f49b7ca..34287fd142f4 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -4422,37 +4422,53 @@ static bool CleartypeSettingChanged()
   return true;
 }
 
-// The main windows message processing method.
-bool nsWindow::ProcessMessage(UINT msg, WPARAM &wParam, LPARAM &lParam,
-                                LRESULT *aRetValue)
+bool
+nsWindow::ExternalHandlerProcessMessage(UINT aMessage,
+                                        WPARAM& aWParam,
+                                        LPARAM& aLParam,
+                                        MSGResult& aResult)
 {
-  // (Large blocks of code should be broken out into OnEvent handlers.)
-  if (mWindowHook.Notify(mWnd, msg, wParam, lParam, aRetValue))
+  if (mWindowHook.Notify(mWnd, aMessage, aWParam, aLParam, &aResult.mResult)) {
     return true;
+  }
 
+  if (IMEHandler::ProcessMessage(this, aMessage, aWParam, aLParam,
+                                 &aResult.mResult, aResult.mConsumed)) {
+    return true;
+  }
+
+  if (MouseScrollHandler::ProcessMessage(this, aMessage, aWParam, aLParam,
+                                         &aResult.mResult, aResult.mConsumed)) {
+    return true;
+  }
+
+  if (PluginHasFocus()) {
+    bool callDefaultWndProc;
+    MSG nativeMsg = WinUtils::InitMSG(aMessage, aWParam, aLParam, mWnd);
+    if (ProcessMessageForPlugin(nativeMsg, &aResult.mResult,
+                                callDefaultWndProc)) {
+      aResult.mConsumed = !callDefaultWndProc;
+      return true;
+    }
+  }
+
+  return false;
+}
+
+// The main windows message processing method.
+bool
+nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
+                         LRESULT *aRetValue)
+{
 #if defined(EVENT_DEBUG_OUTPUT)
   // First param shows all events, second param indicates whether
   // to show mouse move events. See nsWindowDbg for details.
   PrintEvent(msg, SHOW_REPEAT_EVENTS, SHOW_MOUSEMOVE_EVENTS);
 #endif
 
-  bool eatMessage;
-  if (IMEHandler::ProcessMessage(this, msg, wParam, lParam, aRetValue,
-                                 eatMessage)) {
-    return mWnd ? eatMessage : true;
-  }
-
-  if (MouseScrollHandler::ProcessMessage(this, msg, wParam, lParam, aRetValue,
-                                         eatMessage)) {
-    return mWnd ? eatMessage : true;
-  }
-
-  if (PluginHasFocus()) {
-    bool callDefaultWndProc;
-    MSG nativeMsg = WinUtils::InitMSG(msg, wParam, lParam, mWnd);
-    if (ProcessMessageForPlugin(nativeMsg, aRetValue, callDefaultWndProc)) {
-      return mWnd ? !callDefaultWndProc : true;
-    }
+  MSGResult msgResult(aRetValue);
+  if (ExternalHandlerProcessMessage(msg, wParam, lParam, msgResult)) {
+    return (msgResult.mConsumed || !mWnd);
   }
 
   bool result = false;    // call the default nsWindow proc
@@ -4467,6 +4483,7 @@ bool nsWindow::ProcessMessage(UINT msg, WPARAM &wParam, LPARAM &lParam,
     return true;
   }
 
+  // (Large blocks of code should be broken out into OnEvent handlers.)
   switch (msg) {
     // WM_QUERYENDSESSION must be handled by all windows.
     // Otherwise Windows thinks the window can just be killed at will.
diff --git a/widget/windows/nsWindow.h b/widget/windows/nsWindow.h
index 74fd7fe701ab..ff3410e81b07 100644
--- a/widget/windows/nsWindow.h
+++ b/widget/windows/nsWindow.h
@@ -58,6 +58,7 @@ namespace mozilla {
 namespace widget {
 class NativeKey;
 class ModifierKeyState;
+struct MSGResult;
 } // namespace widget
 } // namespacw mozilla;
 
@@ -72,6 +73,7 @@ class nsWindow : public nsWindowBase
   typedef mozilla::widget::WindowHook WindowHook;
   typedef mozilla::widget::TaskbarWindowPreview TaskbarWindowPreview;
   typedef mozilla::widget::NativeKey NativeKey;
+  typedef mozilla::widget::MSGResult MSGResult;
 public:
   nsWindow();
   virtual ~nsWindow();
@@ -328,6 +330,9 @@ protected:
   void                    RelayMouseEvent(UINT aMsg, WPARAM wParam, LPARAM lParam);
   virtual bool            ProcessMessage(UINT msg, WPARAM &wParam,
                                          LPARAM &lParam, LRESULT *aRetValue);
+  bool                    ExternalHandlerProcessMessage(
+                                         UINT aMessage, WPARAM& aWParam,
+                                         LPARAM& aLParam, MSGResult& aResult);
   bool                    ProcessMessageForPlugin(const MSG &aMsg,
                                                   LRESULT *aRetValue, bool &aCallDefWndProc);
   LRESULT                 ProcessCharMessage(const MSG &aMsg,
diff --git a/widget/windows/nsWindowDefs.h b/widget/windows/nsWindowDefs.h
index 56200d9572a1..e3b7e2659d57 100644
--- a/widget/windows/nsWindowDefs.h
+++ b/widget/windows/nsWindowDefs.h
@@ -221,6 +221,28 @@ struct TITLEBARINFOEX
 };
 #endif
 
+namespace mozilla {
+namespace widget {
+
+struct MSGResult
+{
+  // Result for the message.
+  LRESULT& mResult;
+  // If mConsumed is true, the caller shouldn't call next wndproc.
+  bool mConsumed;
+
+  MSGResult(LRESULT* aResult = nullptr) :
+    mResult(aResult ? *aResult : mDefaultResult), mConsumed(false)
+  {
+  }
+
+private:
+  LRESULT mDefaultResult;
+};
+
+} // namespace widget
+} // namespace mozilla
+
 /**************************************************************
  *
  * SECTION: macros