Clarification on the application of window pre-FFT
Generally, any window (rectangular or Von Hann) convolves the input with the transform of the window.  

The transform of a rectangular window is a Sinc shaped "ringy" waveform with infinite ripples, but these side lobes are invisible for pure sinusoids that are exactly integer periodic in the FFT width.  

Smoother windows have lower ripples, but in exchange have wider main lobes.