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