With Halion you can’t dock floating windows.
In a docked window when you click the “down triangle” icon for windows options, you have to select split | or split –
This will split the current window vertically or horizontally. You can then add editors via the “+” tab.
Editor tabs can however be dragged between docked windows. The orange border indicates where the tab will be dropped. A thick edge shows that a new split will be created. Thin edge all round means a tab will be added to the current docked window.