Hello Rui,
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,
Georgi Tunev
the Telerik team
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