BCGControlBar Pro for MFC
UINT BCGM_ON_ERASE_COMPACT_SCROLLBAR

The framework sends BCGM_ON_ERASE_COMPACT_SCROLLBAR registered message to the scroll bar's owner when background of the hidden scroll bar need to be erased.

Parameters
wParamcontrol ID of scroll bar.
lParama pointer to CBCGPScrollBar class.
Returns
the return value is ignored.

You should process this message to implement owner draw toolbars.