I've performed a couple of tests and indeed there is a problem with the load on demand of the RadListBox when it's placed in a container which is initialy not visible. A possible workaround for this is to call the "_initLoadOnDemand()" method once the container is made visible.
I've prepared a sample page demonstrating the suggested workaround.
As a token of gratitude for pointing this problem your Telerik points are updated.
All the best,
the Telerik team
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