EDIT: Thankfully, they have released a new version where the source code is present, it’s in a file named “desktop-release-1.0.1.zip”. Seemingly, source code used to be unavailable for pre-release versions and was missed on v. 1.0.0, which isn’t too strange in all fairness.
Fyi, it’s not FOSS. No source code available.https://github.com/ProtonMail/inbox-desktop
EDIT: Thankfully, they have released a new version where the source code is present, it’s in a file named “desktop-release-1.0.1.zip”. Seemingly, source code used to be unavailable for pre-release versions and was missed on v. 1.0.0, which isn’t too strange in all fairness.