ViewportSource

import type { ViewportSource } from 'pixi-silk/ui/layout';

type ViewportSource = 
  | Application
  | {
  screen: ViewportRect;
  off: unknown;
  on: unknown;
}
  | {
  value: ViewportRect;
  subscribe: () => void;
}
  | ViewportRect;

What a SmartContainer follows: an Application or a renderer (it updates on their resize), a signal of a rectangle (it updates on change), or a plain rectangle (call updateContainer()).