Fix the bell sound when Alt+key is pressed. (#1006)

src/cascadia/WindowsTerminal/IslandWindow.cpp

@@ -162,6 +162,13 @@ LRESULT IslandWindow::MessageHandler(UINT const message, WPARAM const wparam, LP
             return 0; // eat the message
         }
     }
+    case WM_MENUCHAR:
+    {
+        // GH#891: return this LRESULT here to prevent the app from making a
+        // bell when alt+key is pressed. A menu is active and the user presses a
+        // key that does not correspond to any mnemonic or accelerator key,
+        return MAKELRESULT(0, MNC_CLOSE);
+    }
     }
 
     // TODO: handle messages here...