GUIRegisterMsg() subclasses an AutoIt GUI. Ie. a window created with GuiCreate(). A few issues are related to GUIRegisterMsg(): It cannot subclass a control Some messages cannot be handled Only one function for a specific message These issues cannot be described as errors. They are probably consequences of a design choice to limit the complexity of the function. A few issues are related to the subclassing technique in general: _WinAPI_SetWindowLong() Th