Preview an app at phone width, with its code beside it

Preview's bar has a desktop and a phone button: the phone button shows the app 400px wide. In that view, Code opens the page's file next to the phone, and any file you choose in the sidebar opens there instead of in Files. Save with ⌘S and the phone updates within seconds, so you can work on the mobile layout without leaving Preview. The width and Code are remembered for each app and environment.