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RadSplitter for ASP.NET AJAX
This can be achieved by using the RadAjaxLoadingPanel control.
NOTE: This feature is supported out of the box since Q1 2011. The corresponding property is ShowContentDuringLoad and is shown in this online demo.
In this manner you will ensure that when the external page starts loading in the RadPane, the loading panel will be shown and when the loading ends, it will hide.
You can find a working demo in the attached archive file. In this demo, the time taking loading of the content page is simulated by the Thread.Sleep method in its code behind.
Please, note, that this approach concerns only scenarios in which the main page and the content page are from the same domain due to security reasons.