Z-Order

The z-order of a window indicates the window's position in a stack of overlapping windows. This window stack is oriented along an imaginary axis, the z-axis, extending outward from the screen. The window at the top of the z-order overlaps all other windows. The window at the bottom of the z-order is overlapped by all other windows.

The system maintains the z-order in a single list. It adds windows to the z-order based on whether they are topmost windows, top-level windows, or child windows. A topmost window overlaps all other non-topmost windows, regardless of whether it is the active or foreground window. A topmost window has the WS_EX_TOPMOST style. All topmost windows appear in the z-order before any non-topmost windows. A child window is grouped with its parent in z-order.

When an application creates a window, the system puts it at the top of the z-order for windows of the same type. You can use the BringWindowToTop function to bring a window to the top of the z-order for windows of the same type. You can rearrange the z-order by using the SetWindowPos and DeferWindowPos functions.

The user changes the z-order by activating a different window. The system positions the active window at the top of the z-order for windows of the same type. When a window comes to the top of z-order, so do its child windows. You can use the GetTopWindow function to search all child windows of a parent window and return a handle to the child window that is highest in z-order. The GetNextWindow function retrieves a handle to the next or previous window in z-order.

https://msdn.microsoft.com/en-us/library/windows/desktop/ms632599(v=vs.85).aspx#zorder