Viewport and window separation

During runtime, I am looking for direction on creating essentially an editor view.
I need to allow for the application window to scale (shrink, grow, maximize) but have the viewport window within it set to a specific size and scale and resolution within the window.

I do not wan’t to fake this by adding a widget over the entire viewport and then use a render target image.
it needs to be the actual viewport so that i can still do mouse clicks, and deproject screen to world.
I do not want an additional widow which is separate from the application window.

In the end it should essentially look like unreal editor, where ui is on the sides with the viewport in the middle.
Does anyone have a solution?