diff --git a/widget/src/cocoa/ComplexTextInputPanel.h b/widget/src/cocoa/ComplexTextInputPanel.h
index 80a247cc4fa..4f861c48a23 100644
--- a/widget/src/cocoa/ComplexTextInputPanel.h
+++ b/widget/src/cocoa/ComplexTextInputPanel.h
@@ -38,6 +38,7 @@
 
 - (NSTextInputContext*)inputContext;
 - (BOOL)interpretKeyEvent:(NSEvent*)event string:(NSString**)string;
+- (void)cancelComposition;
 
 @end
 
diff --git a/widget/src/cocoa/ComplexTextInputPanel.mm b/widget/src/cocoa/ComplexTextInputPanel.mm
index af6ba3e4f68..05965632f5e 100644
--- a/widget/src/cocoa/ComplexTextInputPanel.mm
+++ b/widget/src/cocoa/ComplexTextInputPanel.mm
@@ -83,8 +83,7 @@
 
 - (void)keyboardInputSourceChanged:(NSNotification *)notification
 {
-  [mInputTextView setString:@""];
-  [self orderOut:nil];
+  [self cancelComposition];
 }
 
 - (BOOL)interpretKeyEvent:(NSEvent*)event string:(NSString**)string
@@ -119,4 +118,10 @@
   return [mInputTextView inputContext];
 }
 
+- (void)cancelComposition
+{
+  [mInputTextView setString:@""];
+  [self orderOut:nil];
+}
+
 @end
diff --git a/widget/src/cocoa/nsChildView.mm b/widget/src/cocoa/nsChildView.mm
index 57790ba1cff..e8974933cfb 100644
--- a/widget/src/cocoa/nsChildView.mm
+++ b/widget/src/cocoa/nsChildView.mm
@@ -5365,9 +5365,15 @@ static const char* ToEscapedString(NSString* aString, nsCAutoString& aBuf)
       mGeckoChild->DispatchWindowEvent(pluginEvent);
       if (!mGeckoChild)
         return;
-      
+
       if (!mPluginComplexTextInputRequested) {
-        // Ideally we'd cancel any TSM composition here.
+#ifdef NP_NO_CARBON
+        [[ComplexTextInputPanel sharedComplexTextInputPanel] cancelComposition];
+#else
+        if (mPluginTSMDoc) {
+          ::FixTSMDocument(mPluginTSMDoc);
+        }
+#endif
         return;
       }