How to make an lv_win a child of another lv_win

How-to
2

Partially solved: As soon as the background container (the lv_win_t itself) had this removed

    lv_obj_set_flex_flow(win, LV_FLEX_FLOW_COLUMN);

which in turn called

    lv_obj_set_style_flex_flow(obj, flow, 0);
    lv_obj_set_style_layout(obj, LV_LAYOUT_FLEX, 0);

then child windows started occupying the whole display area instead of being placed below the header (title bar) of the parent window. Note: positioning and sizing of the “content area” had to be done manually since the FLEX-FLOW COLUMN and FLEX-GROW attribute was no longer governing its position and size.

What I still don’t know is WHEN in the flow of execution does the child window get forced to be below the parent window’s header (title bar)? Is it in the DRAWING? Or perhaps refreshing child object layout before that? I’m guessing the latter.