Rust hybrid application, even though the application itself seems not ready for publication yet. I think they do their best to find a middle ground and do try a lot of different approaches in tech, like native for some parts (AndroidMail, iOS-Mail, Drive ...) or some hybrid form (Android, iOS and Web Proton Pass with a Rust / native hybrid by using Mozilla's UniFFI or Flutter-Rust hybrid for Proton Wallet). What they use for Proton Calendar is sadly less known.
@Techaltar @protonprivacy One can see the web overlay and could use that in e.g. an Ionic application (like they do on Linux with the Proton Mail + Calendar app) or maybe Tauri would work. Also, an old fork of the calendar UI for Android in Kotlin is public and some old React Native version (?). Proton does take user requests systematically with UserVoice.