As an explicit design choice, there are no stock lumino layouts that will create an independently draggable window. Part of this is that it demands a whole separate layer of concerns for keyboard navigation and window layout restoring.
However, an extension has full access to do whatever it needs to the underlying DOM, and as such could basically whatever is needed.
For example, jupyterlab-tour uses react-floater to put on-demand popups on the page, but these are not draggable, but have good keyboard reactivity.