Yeah I can kinda hear/see what you're talking about now... not sure what the deal is exactly. I've definitely experienced a good amount of weird aliasing artifacts in the spectral processes before... part of me wonders if this could be fixed if maybe we had control over the window shape (PVOC only uses a Hamming window) though that may have absolutely nothing to do with anything, lol. Maybe setting a higher value for the window overlap parameter may do something? I think it's already 3 by default though, and the highest is 4, so idk. Very curious indeed.
And that is the spectrogram in Renoise, I have it open pretty much all the time now. On my laptop I use VB Cable (
https://www.vb-audio.com/Cable/index.htm) which you can set the "Cable Input" as your primary output in system settings, then set "Cable Output" as a Line Input within Renoise, and this way you can monitor the spectrogram of whatever you're listening to, including any cdp stuff. I highly recommend! (on my home desktop I forego the VB Cable stuff because I have an 8 i/o interface which can do the same thing with less latency.)
(Oh yes, I just remembered you use Linux, so VB Cable prob isnt an option. You can prob do the same thing using JACK audio server though!)