PIO Core: build offline (w/o Internet connection)

I haven’t tried it with a simple program with no external library dependencies. (I work only on a small number of open source projects, so I don’t have a LOT of PIO experience.) As I’m typing this, I have a terrible Internet connection, and I’m seeing two things when I start PIO:
In the “Output” tab:

Installing PlatformIO IDE...
It may take a few minutes depending on your connection speed
Please do not close this window and do not open other folders until this process is completed.

And in a little window in the lower right corner: PlatformIO Installer: Downloading portable Python interpreter.

I was working in PIO last night - why is it “Installing PlatformIO IDE” from some online source, and “downloading a portable Python interpreter” now, just a few hours later? It’s as if it thinks I want to update everything every time I start PIO.

It just now finished, and I see this in the Output tab:

PlatformIO IDE installed successfully.

Please restart VSCode.

So what did it just have to install that requires a VSCode restart?