Problem with setParent() for a child of a QDialog embedded in a QGraphicsView

With the attached example program, I get the following behavior:

== First round ==

• start the program
  => output
  combo box constructor
  list widget constructor
• click the check box => the combobox appears
• click the check box again => the combobox disappears
• close the dialog
  => no more output

This is correct, as finally the combo box does not have a parent which
destroys it

== Second round ==

• start the program
  => output
  combo box constructor
  list widget constructor
• click the check box => the combobox appears
• click on the combobox => the dropdown appears
• click again => the dropdown disappears
• click the check box again => the combobox disappears
• close the dialog
  => output
  list widget destructor

This is not correct. The combo box does not have a parent, and the list
widget is a child
(or grandchild) of the combo box and should at this moment be independent
of the graphics view.

In a real world program this can lead to crashes due to widgets which are
supposed to be
recycled over multiple instantiations of a graphics view being destroyed
behind the
applications's back (note: the subclassing of QListWidget and QComboBox in
the example
program is just for the purpose of printing debugging output. The behavior
is independent
of this).

The reason for this behavior is either in the hide_helper() of QWidget or
in the setParent().
Showing a child of a dialog embedded in a graphics view automatically
creates a corresponding
proxy widget. Then, either hiding this child should destroy the proxy
widget, or the setParent()
of the parent dialog should do this.

This was tested on both Qt 4.4.x and Qt 4.5.0 on X11.