The parent-child relationship and ownership relationship between windows are two kinds of relationship, and the correlation is not big. Why can't a child window be an owner window? ComboBox's drop-down list window is not a child window. Who is its owner?
Because the top-level window can only have an owner window (it is not necessary, it can be null).
WS_ Child’s window is a child window, and the one without it is the top-level window. So the child window can only set the parent window.
The drop-down list window should be a child window, so it has only parent and no owner.