BCGControlBar Pro for MFC
UINT BCGM_CHANGING_ACTIVE_TAB

The message is sent to the parent of CBCGPTabWnd object.

Parameters
wParamcontains the number of a tab that is being activated.
lParama 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.