i want to subclass NEtmeeting Chat window so that whenever new messages comes in.. the window starts flashing...

I surprised when i saw that there was no message posted when the contents of an Edit Window (Textbox etc) changed.. when i replaced the whole text WM_PAINT got posted...When I typed in the text WM_Char, WM_KEYXXX were posted...
If there is no message associated with the change of contents how come the Textbox implement a Change Event???