Resizing problem in design mode

[roland]

I made a usercontrol that inherits from System.Web.UI.WebControls.Panel.
Because it has to provide a number of client-side services, the controls
inside the panel are instantiated at Render time.
I made a Designer that inherits from ControlDesigner to render this control
in design mode. So far so good.
But:
When I resize this control in design mode the following happens: the
rendering of the new size is overlapping the former one, the old rendering
is still visible. On top of it, the handles of the control are not following
the new size, they are staying (almost) on their former position: when you
are making the control smaller, the handles are following at glacial speed.
I checked the rendering process in debug mode and verified that the
rendering is only called once. The same happens when I change the size in
the property page: the handles don't move but the controls inside are
adapted.
How can I force that the former rendering is cleared before I render the new
one? How can I ensure that the designer is aware of the new settings?
Any hints are highly appreciated. Thank in advance,
Roland