A follow-up question: the role of the "-owner" option is not clear to me. Why and when is it necessary, and why does it fix the problem?

It is necessary whenever you're using the functional interface

If you had used $Window->BrowseForFolder() it would have happened automagically

It lets the windowing system/ window manager, decide correctly to allow children of parent to display on top of parent , in effect it propagates the WS_EX_TOPMOST flag to the children