LuaRT - Resizeable UIs

For an example, A UI or a 'Background' could be resized using the mouse cursor & instead of setting a local position.

It's already the case. Window widget provides several window styles :

"dialog" : the default, a resizeable dialog. The user can maximize/minimize the window.
"fixed" : a fixed size dialog.
"float" : a tiny float window.
"single" : a fixed size dialog with system menu (only minimize-able).
"raw" : a fixed size without title bar or borders.

Samir Thank you for the clarification.

Samir Is there also anyway to run the UI in the center of the monitor or the screen?

Yes all widgets have a center() method.
For example, to center a Window, you need to call the Window:center() method.