The demo is built using client-side functionality only so you will need to develop custom logic in order to save the state during a postback. For example you can modify the demo and write the state (size, position, Url, etc) of the controls (windows / tabs) in a hidden field and to re-apply these settings again after the postback.
All the best,
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items