An infinitely long sinusoid has a Fourier transform that is non-zero at <u>only</u> one frequency.  Multiplying that sinusoid by <u>any</u> window function is a non-linear operation that <u>always</u> creates new Fourier transform components.  That is "leakage", whether or not the number of cycles in the window length is an integer.  If that Fourier transform is a sinc function (rectangular window) and we sample it at only the main lobe and the zero-crossings, then we will see the illusion of no leakage.  But we will be quickly disillusioned when we apply that rectangular window to our sinusoid embedded in a summation of other signals.