Use the Read Only command to make the current window
  read-only. Once a window is made read-only, all of the
  editing commands──Undo, Cut, Copy, Paste, and Clear──
  are inactive and text entry is not permitted.
 
  The Read Only command is a toggle that turns the option
  on and off. Whenever the read-only option is on, an "R"
  appears in the status bar at the bottom of the screen
  and a bullet precedes the command in the menu:
 
    ┌─────────────────────┐
    │• Read Only          │
    └─────────────────────┘