Yeah, just having it’s own terminal would be enough… then at least you could see what it’s doing, and possibly abort if you don’t want it download stuff at that point in time. As Ivan pointed out above… for IntelliSense to function, you need
a real toolchain and related frameworks to build configuration for IntelliSense service.
So if you don’t have it installed, either because you haven’t used that framework before on the machine in question, or because you uninstalled it for some reason… it’s gunna get downloaded during the IntelliSense rebuild in order for it to actually function. But yeah, some way of knowing what it’s doing would be nice 
You’ll often see a rebuild when you edit your platformio.ini… and if you add a new lib_dep… you’ll spot it downloading the library during that scan.