Which GUI library is used in latest Calibre?

I just installed latest Calibre from source on Ubuntu, by provided snippet on Calibre home page, and was very pleasantly surprised by Calibre start-up time. Almost immediate. I noticed that Qt UI is changed by something other but I can't figure out what is it and by looking in the code it seems Qt is still used. I noticed also that some other apps like Serna XML editor uses same UI. Can someone tell me which UI library is used and when this change happened in Calibre? I read changelog but didn't noticed anything related to GUI

Thanks