Added note.

src/win32_window.c

@@ -684,6 +684,8 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
                 updateClipRect(window);
             }
 
+            // NOTE: This cannot use LOWORD/HIWORD recommended by MSDN, as
+            // those macros do not handle negative window positions correctly
             _glfwInputWindowPos(window,
                                 GET_X_LPARAM(lParam),
                                 GET_Y_LPARAM(lParam));