Thank you for contacting us. Unfortunately this scenario is currently not supported and we are not aware of an existing workaround.
The GroupPickerItemTemplate property is set to the ItemsTemplate property of a control named JumpListGroupPicker. JumpListGroupPicker is basically a System.Windows.Controls.ItemsControl. As far as I know, changing the ItemsPanel of a System.Windows.Controls.ItemsControl is not yet available in Windows Phone 7. The same scenario has been working properly in WPF. Some time ago the issue was present in Silverlight as well but Microsoft managed to fix it. The problem is that the fix has not been included in the last Windows Phone release.
We will be able to achieve this scenario as soon as we manage to integrate our ListBox into the JumpList. Until then, there is not much we can do. Sorry for the inconvenience caused.
the Telerik team
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