Under the list of controls that will adjust with the device detection screen sizes and device types, the RadPanelBar is not listed. It would be handy if this control was able to automatically layout differently if there were more space. For example, on a wide screen monitor, the panels would be side by side, if there was room. And if the same page was viewed in a smaller screen size, the panels would stack and become collapsible as they are by default now.
While it may be possible to adjust the panel bar for a set number of root items, achieving such behaviour for most scenarios would be virtually impossible due to how panelbar works. For instance, if RadPanelBar is not in FullExpandMode, the individual root items will cancel each other out. Also, determining the height of each root item is going to be quite difficult, if not impossible. Perhaps, this particular niche is better suited for a control we don't already have, say a card view or something similar.