Here is the Docked mode option for "Preview" window.
I removed the docked mode i.e. undocked it. And here is what I got.
There is no more "Docked Mode" option. Now I cannot properly work with designing layouts. Preview automatically hides when I change anything in the layout.
Here is the simple solution for reverting to "Docked Mode".
There you go. The Preview window is in Docked Mode. And there is the option for Docked Mode again.