Three ways to move the 3D out of its fixed slot atop the right rail. Each opens a faithful editor mockup — drag the box to rotate, drag the fold slider, try the placement. Stand-in CSS box (the real one is the three.js preview).
A draggable window (defaults top-left) that floats over the canvas and snaps to the nearest corner when you let go. Minimize/expand buttons. The right rail is freed entirely for controls.
A compact viewer parked in a canvas corner. One click collapses it to a small “▣ 3D preview” pill so you get the full flat dieline when you don’t need the box. Rail freed.
The main area splits: flat dieline left, 3D right, with a draggable divider and a 2D / Split / 3D toggle up top. The biggest, most legible 3D — a proper CAD-style two-view. Rail freed.
My pick: V2 corner-dock for the least disruption, or V3 split if you want the 3D to feel first-class. V1 is the most flexible but adds a “where did my window go” cost. Nothing in the app changed for these — they’re standalone mockups on your Desktop.