The xamDockManager will fill the other area but as discussed in several other threads here the panes within the xamDockManager are not designed to fill the available area - instead they do as occurs in VS where the docked panes are docked to the edges around a central content which in VS is the mdi client and in xamDockManager can be any element. The only element that you can put into the content that would still participate in dragging, etc. would be a DocumentContentHost to mimic the VS tabbed document area. The next release will include the ability to have the docked panes fill the available area.