I've seen this request a couple of times before but it seems like each person has a different concept of what that would mean - i.e. does it prevent docking, does it prevent pinning/unpinning, does it prevent resizing of splitters (all or just some - e.g. resizing the unpinned flyout would be ok), can the user still float something explicitly through the commands (e.g. context menu), etc. So currently there is no property to lock the layout per se.

If you just want to disable docking completely then you could use the GetPanes method to enumerate the ContentPanes and set their AllowDocking property to false (or create your own attached property and bind the AllowDocking to that as you create each ContentPane). If you just wanted to disable dragging but still allow the user to toggle the state from floating to docked, you could instead try handling the PaneDragStarting and setting e.Cancel to true.