since the layout structure of widgets in a frame is a tree,it will be more convenient by construct the layout by drag widgets in the widgets structure view in the top-right corner of the designers default layout.

Added:
1. Draging from FORM onto object inspector is work if the FORM is a QWidget.
move from one tab in a QTabWidget into another is not work.
please try to drag tipTab11 into tab2 in the attatched prj file.
2.My suggestion is to enable the way to drag from the object inspector then into the object inspector.