WM_NCACTIVATE 
The WM_NCACTIVATE message is sent to a window when its nonclient area needs to
be changed to indicate an active or inactive state.
WM_NCACTIVATE
fActive = (BOOL) wParam; // new state of the title bar or icon
Parameters
fActive
Value of wParam. Specifies when a title bar or icon needs to be changed to indicate an active
or inactive state. If an active title bar or icon is to be drawn, the fActive parameter is TRUE. It is FALSE for an inactive title bar or icon.
Return Values
When the fActive parameter is FALSE, an application should return TRUE to indicate that
Windows should proceed with the default processing, or it should return FALSE to
prevent the title bar or icon from being deactivated. When fActive is TRUE, the return value is ignored.
Default Action
The DefWindowProc function draws the title bar or icon title in its active colors when the fActive parameter is TRUE and in its inactive colors when fActive is FALSE.
See Also
DefWindowProc