The parent-child relationship and the 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 owns it?
Because a top-level window can only have an Owner window (it is not necessary, it can be NULL).
WS_CHILD’s window is a sub-window, and what is not is the top-level window. Therefore, the child window can only set the Parent window.
The drop-down list window should be a child window, so only Parent, not Owner.