Now drag the Properties pane to the center section of the group of boxes as shown, and note the highlighting encompasses the entire HTML pane:

Notice the Properties pane fills the HTML pane area, and is the currently shown pane. If you click the HTML: (ID=1656), the HTML pane becomes the active pane.

 

--

You may now drag either of these panes out to a floating position, by dragging as shown:

--

 

--