[5.4.0b15 / W10 64] Unity initially opens Mono AND VS every time editor is relaunched

I don't know if this is related to the Unity version or not, but it's a new issue with this install to me. The first time I double-click a script to open it after every editor launch, I get both VS and Mono open. Unity-VS integration seems fine otherwise, and after I close that instance of Mono it doesn't reopen until I relaunch the Unity editor. But it slows down the process and it's very annoying. I don't have a separate Mono instance to uninstall, VS is set as the code editor in preferences and in Windows file associations for .cs and everything else. I can't find any specific trigger that might be launching Mono. Anyone else running into this or have thoughts?