What I would suggest you is to observe for changing the focus and keep track in your ViewModel which was the last active pane and make the RibbonBar work for it. I know this is not very elegant way to achieve the goal, but the Docking control doesn't support concept for activation of panes yet, which is in our TODO list.
About the problem you are observing - could you please prepare and send us a sample project that reproduces the problem? This would help us investigate what the problem is and help you resolve it.
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