I've been going over the Viewport documentation and tutorials repeatedly, as well as searching all over the Internet, but I am left with plenty of questions.

Can I use a Viewport to render to without it appearing on screen, or messing up any UI layout, or taking any mouse focus?

I think the answer is "yes," and that this is what is called a render target. How do I use a Viewport as a render target (in GDScript). I think the tutorial regarding using a Viewport as a texture is sort of discussing this. But, I see that they put the Viewport into a scene. Why does it need to be in a scene if it's not going to be visible? What scene does it need to be in? Can it be in a bogus scene not currently active?

If it has to be a part of the currently active scene, how do I ensure it doesn't mess up the other components of that scene?

Can I use a Viewport to render to without it appearing on screen, or messing up any UI layout, or taking any mouse focus?

That makes me think you are using a ViewportContainer. Those aren't needed to render viewports. Just the Viewport node should suffice. If that's not the case, then my second answer below should help.

If it has to be a part of the currently active scene, how do I ensure it doesn't mess up the other components of that scene?

By setting its own_world property to true. If you do that, it shouldn't be affected by the rest of the scene; only its children.

Thanks. I've added the viewport to my scene, and set own_world true, and it's working now.

3 years later