Focus mode clears the screen down to the page. It is a view, not a mode of the document: nothing about the script changes.
Turn it on
Press ⌘⇧., or pick Enter Focus Mode from the command palette. The toolbar and both sidebars fold away and the page centers; a small reminder in the corner says how to leave.

Focus mode on and off: the chrome folds away and the page centers, then Esc brings everything back where it was. Reach the sidebars without leaving
⌘⇧\ and ⌘\ still work in focus mode, so you can peek at the outline or a skill result and close it again.
Turn it off
Press ⌘⇧. again, or ⎋ with nothing else open. The layout returns to how you left it.