A previously unforced thunk in an attribute set does not lead to indentation when it won't evaluate to a nested structure:
  nix-repl> :p let x = 1 + 2; in [ { inherit x; } { inherit x; } ]
  [
    { x = 3; }
    { x = 3; }
  ]

Same for a list:
  nix-repl> :p let x = 1 + 2; in [ [ x ] [ x ] ]
  [
    [ 3 ]
    [ 3 ]
  ]