UINT BCGM_CHANGING_ACTIVE_TAB

The message is sent to the parent of CBCGPTabWnd object.

Parameters

wParam	contains the number of a tab that is being activated.
lParam	a pointer to CBCGPBaseTabWnd window (sender).

Returns

Should return nonzero to abort processing tab change, or zero to continue.

You should process this message to receive notifications from CBCGPTabWnd objects when a tab is being activated.