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Solution by AvS (6-Jan-2007):
After you created the Tab_Control, add the following line:

SetWindowLong (Handle (Base_Window_Type (Tab_Control)), GWL_EXSTYLE,
GetWindowLong (Handle (Base_Window_Type (Tab_Control))) +
WS_EX_CONTROLPARENT);

The discussion here is about how to integrate it properly into GWindows:
http://sourceforge.net/mailarchive/message.php?msg_id=37837306