"Mailspring could not find the mailsync process" in dev build

I’ve got a copy of the Mailspring codebase on two PCs (my personal PC and my work PC). On the first one, I managed to run properly after quite a bit of pain (couldn’t log in… ended up having to use “Run with Debug Flags” with a production version, log in there, exit it, manually kill lingering mailsync.exe processes, then I could start my dev build properly. I posted that here: "Signing in" forever in GitHub dev build).

On my second PC, I’m getting this error even after running npm install and haven’t figured out how to progress past it:

Mailspring could not find the mailsync process. If you’re building Mailspring from source, make sure mailsync.tar.gz has been downloaded and unpacked in your working copy.

The thing is I’m not even sure where to get the mailsync.tar.gz from, or whether that’s appropriate for Windows.

I’m using the current master commit 187ba07206f9952bfe70f0d09779689a5bc98df3 as of the time I’m posting this.

Windows 10, version 20H2
Node v15.12.0
npm v7.6.3

Any ideas?

@bengotow I think we need you on this one!