In the DirectDraw mode "Primary surface" Hook is not activated, if the application is started from a debugger. Thus representation errors are possible (e.g. when shifting the window).
The reason is an overlap of the Hook functions of the debugger and the FALCON/EAGLE driver. The debugger uses the new Hook chain before the FALCON/EAGLE driver can initialize the Hook chain completely. If the FALCON/EAGLE Hook chain would be activated parallel to the Hook chain of the debugger, the system could crash.
