ACUCOBOL-GT treats each mouse action like a unique function key that the program can act on. Each (unmasked) mouse action returns an exception value to the program. By examining the exception value, your program can determine which action just occurred.
Because it would be very disruptive to your program to return exception codes every time the user moved the mouse, the mouse actions can be masked (ignored). By default, all mouse actions are masked (except those that are managed by automatic mouse handling). So, your program is not interrupted by any mouse action. You can enable some or all of the mouse actions in any combination by setting the COBOL configuration variable MOUSE-FLAGS. This allows you to program only for those mouse actions that you care about. See Unmasking Mouse Actions for more information.
After your program becomes aware of a mouse action, you'll need additional information, such as the location of the mouse. ACUCOBOL-GT provides a utility library routine called W$MOUSE that enables you to determine the mouse's location and the state of each of its buttons. You can also use this routine to control mouse behavior. You'd typically call W$MOUSE in response to the user's pressing one of the buttons, to determine where the mouse is located.