Hi!
Let me heath up an old topic. At least the one that I was mentioned many times before. 
480 px Viewport size preview is unrealistic. As long as X/Pro has it, people will build broken mobile sites. I see that literally all the time.
360px, 375px are more realistic, and the preview needs to check common 320px too.
Iโd really like to see some solution for this one day. Maybe a sub-indicator next to the 480px button, that would allow narrowing the preview to 360 and 320px?
Thanks!