CreateDocument@PaletteNotebook[DynamicModule[{init, contents},
Dynamic[If[TrueQ@init,
contents,
ProgressIndicator[Appearance -> "Necklace"]
]],
SynchronousInitialization -> False,
Initialization :> (
init = False;
Pause[5]; contents = Framed@Button["Foo"];
init = True;
)]
, WindowMargins -> Automatic, WindowSize -> 300]