When completed a task it automatically removes from the whiteboard, removed also the arrow also and rearranges.
I love how mindomo does it and would love to see the feature here as well.
Mindomo has limitations you have solved, only for big projects a big mindmap that features this would be great. I use these big mindmaps in mindomo cause it gives my a perfect overview of where the project is at in a birdseye perspective.
When it's cluttered with completed tasks or I need to rearrange it myself it loses it's magic.