The second monitor is a phone on a stand

Hotel room, one laptop, no external display. You're iterating on the checkout page.

VS Code runs in the container on display-1; the laptop has it fullscreen. Firefox with the dev server runs on display-2, launched from a terminal session created with display: "2". You open display-2 on the phone with ?readonly=true&toolbar=false and prop it against the lamp.

Save in the editor. The dev server reloads. The phone shows the new render. It isn't a screenshot and it isn't a viewport emulation: it's a real Firefox window on the server, rendered on a second screen that happens to be a phone.

Need DevTools? Open display-2 on the laptop as well. Same window, same state, two views.

A container can run as many displays as you give it, one per application.

Each display is its own URL, so any screen can be any monitor.