I was thinking before about enabling this feature for different reasons but was focusing on other things. @KAepora has recently made a good point in the An Analysis of the ProtonMail Cryptographic Architecture paper about a need to run webclient locally in order to enhance code integrity. Given that point and that the local store feature is substantially completed I think the time has come to address this issue.

I've already got a proof of concept running on my laptop and it appears to be working well at first glance. So the app is already capable to work with local webclient. I will need to integrate the webclient building into the CI build pipeline. It will clone the code by specifc hash from https://github.com/ProtonMail/WebClient, build it and embed to the app. For now local webclient will be able to talk only to a single API endpoint https://mail.protonmail.com/api, the default one.

There are no plans for enabling built-in webclient automatic updates separately form the app itself. So there will be a need to build a new desktop version in order to update a local webclient. It would be quite easy though to allow advanced users to re-build the webclient in their own and make the app use it since I don't hack/preprocess the original webclient in any way but use it as a completely given static thing.

This feature is going to be enabled initially for Protonmail only. Tutanota has a more complex web client architecture that involves Service Workers and WebSockets logic which makes it more complex for integrating with Electron. Besides, it has been evolving quite rapidly which means there would be a need to release desktop versions continuously to catch up with the web client.

I'd say there is a good chance to include this feature into the final v2 release.

CC @thiswillbeyourgithub