Does spectral leakage include negative frequencies?
Yes spectral leakage includes both negative and positive frequencies; i.e, the convolution is from $-\pi$ to $\pi$.

However its effect depends on what exists at the negative frequency region, when you want to compute the resulting leakage on a given positive frequency.

On a typical application where you have a real valued sine wave of frequency $f_0$ as the windowed signal, then there is a dirac impulse at the negative frequency $-f_0$ as well as the positive frequency $f_0$. Then both impulses shift the spectrum of the window Fourier transform to their own locations and create leakage effects on any other frequency (unless they have zero value at that particular frequency). However the leakage is expected to be smaller from a negative frequency as it's side lobes are already decaying more than the positive frequency contributions. (This can be the opposite when both impulses are close to $\pi$ or close to $0$ radians.)

But if you have a complex signal, such as an analytic signal which has zero negative spectral content, then effectively the negative frequencies will not create leakage.