A window, not a portal. A window that works like an Interdimensional Item Frame.
Place the Dimensional Window on a block in the Overworld at ~ ~ ~ and it shows you what's at ~ ~ ~ in the Nether. Place it in the Nether, and it shows you the Overworld. Live, in real time.
“Shows a real-time view of the corresponding block in the Nether or the Overworld.”
Displayed areas are loaded while being viewed.
How it works
The center of the Dimensional Window (Functional Block) renders the block that exists at the same coordinates in the opposite dimension. The other dimension stays loaded while you view it.
