I added an example where you (should be able to) click a Billboard and then move the mouse to “drag” the Billboard, but I’m not seeing the Billboard move in the viewer even though I can see that its position is getting updated.
We have an example which shows a billboard’s position updating with the mouse’s position: Sandcastle.
Your example is using a separate picked object, which may be a copy of the original billboard.
Thanks @dzung you’re correct! I was able to get it working by searching for the picked object and updating in the collection.