The previous and next buttons become active when there are more DocumentWindows than DocumentTabStrip can display. If you have a small amount of DocumentWindows and they can fit into the width of the strip the previous and next buttons are expected to be disabled:
If you increase the count of the windows, the buttons will enable automatically in order to allow easy navigation through different windows:
If I understand you correctly you mentioned that you have enough windows to allow scrolling. In this case, it is strange why the buttons remain disabled. I attached my sample project for your reference. Could you please refer to it and let me know how it differs from your project. Am I missing something?
Looking forward to your reply.