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 CtrlShift., 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
CtrlShift\ and Ctrl\ still work in focus mode, so you can peek at the outline or a skill result and close it again.
Turn it off
Press CtrlShift. again, or Esc with nothing else open. The layout returns to how you left it.