Currently, the PanelBar does not have a concept for selection. When I click on an item, it looks like "selected" due to the background but there is no way I can save that selection or set it programmatically if I need to retrieve the selected state at some point.
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ADMIN EDIT
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For the time being, you can use the OnItemRender event and some custom CSS to visually mark the desired item as selected: https://blazorrepl.telerik.com/GobwQhly107nUDoZ51.
If you need to get the item that the user last "selected", you can use the OnItemClick event as we mark the item as selected when the user clicks on it.
Maybe my use case is not exactly the same, but it has also to do with selection. I could find information in Telerik documentation about using this bar for navigation - you can bind an UrlField to the PanelBar.
However, it is not really useful if the item with matching Url cannot be selected. Or IMO when using this feature with binding, I would actually expect that corresponding item is auto-selected. And it should also work for child items (with auto-expansion).
Or am I missing something?