This function retrieves the visibility state of the specified window.
Nonzero indicates that the specified window and its parent window have the WS_VISIBLE style. Zero indicates that the specified window and its parent window do not have the WS_VISIBLE style. Because the return value specifies whether the window has the WS_VISIBLE style, it may be nonzero even if the window is totally obscured by other windows.
The visibility state of a window is indicated by the WS_VISIBLE style bit. When WS_VISIBLE is set, the window is displayed and subsequent drawing into it is displayed as long as the window has the WS_VISIBLE style.
Any drawing to a window with the WS_VISIBLE style will not be displayed if the window is obscured by other windows or is clipped by its parent window.