Extension development - enable dragger?

I have been doing some extension development, and have had pretty good success with it. But I'm struggling with one thing: I am trying to develop a 3D widget, with the usual x, y, z, rx, ry, rz properties to position it. The runtime preview respects these attributes as expected, but the design time view has a few issues:

1. The values appear to be applied twice in the design canvas, e.g. if I set x=0.1, the widget is rendered at x=0.2. If I rotate by 30 degrees, the widget is rendered rotated 60 degrees
2. The widget shows the dragger (translate/rotate) control in the design view, and I can use it to apply translations/rotations to the rendered widget, but those changes don't update the widget's location properties. For example, I can drag the widget "up" on the Y axis, and it moves as expected on the design canvas, but the Y property value doesn't change (nor do any other properties change).
3. Clicking the widget in the design canvas highlghts the rendered widget, but doesn't select it in the tree view. To fully activate the widget, you have to select it in the tree view.

I suspect items 2 & 3 are related.

So I guess I've left something out of my design JS file, but I'm not sure what I need to make the design canvas work the way it's supposed to. I've been using some of the OCTO extensions as a guide, particularly the Progress-3D widget.

Can anyone give me some tips on how to make sure the positioning functions in the design canvas work with my extension? @SteveGhee or @RolandRaytchev or @sgreywilson maybe?