Also if you want here’s the Project (UE4.27) that uses popped up windows and a few parts always visible as sort of menu
You can find there some different things that can be done with them (like sticking to sides, moving to front on click, saving location and size and more) as well how it was implemented
(pay good attention on Anchors and all other values below that)
Maybe you’ll get some inspiration or just copy-paste to use in your Project
It is somewhat buggy and some stuff is weirdly implemented but some stuff can be pretty useful
Enjoy! 