Make two separate layouts in Sapper, one for desktop and one for mobile?

Question

Frontend

How to make two separate layouts in Sapper, for desktop and for mobile. In Svelte, I in Main.js check if userAgent and run either const app = new Appmobile or Appdesktop. But not so in Sapper. I tried to write something like this in client.js and in _layout.svelte - it doesn't work.