In the current version of RadAutoCompleteBox control grouping is not supported out of the box and implementing it could lead to unexpected and not supported behaviors. The previously described and provided sample projects were meant to be used as a reference for a simple implementation of the grouping feature. I took a look at the provided project in the thread with ID: 823626
and unfortunately achieving the desired custom navigation behavior is not an easy task and borders to be impossible in the current version of RadAutoCompleteBox. As there is not grouping support in the control in the project from that thread the RadExpander items are treated as simple items from the FilteredItems collection of the AutoCompleteBox. The control does not provide information for the current item which is highlighted and it is impossible to trigger a custom logic which would expand that item (if it is RadExpander).
The described custom implementation of grouping with custom ItemTemplate that contains Expander control is out of the scope of the RadAutoCompleteBox and we cannot provide you with the best approach which could be used to create such a custom control.
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