I noticed in app.js the default mailvelope keyring identifier is set to this.user_id, which is a long unique identifier. This produced an unexpected behavior for me. My use case was as such:
I initially installed Mailvelope, exported my GPG keys from roundcube/enigma and loaded them into Mailvelope (no issues). Then I attempted to encrypt an outgoing email using the Mailvelope api plugin (vice the enigma plugin) and was told that Mailvelope was unable to find the requisite public keys for the intended recipient. Wait, what? I just imported those in the last step... In debugging app.js through the browser I saw that the keyring was in fact empty, yet the only keyring I saw through the plugin interface in Firefox was the default keyring that I had previously imported keys into. It wasn't until I used the mailvelope API to manually open the settings page for the new keyring that I was able to in face see the alternate keyring (and subsequently add keys to it).
As it turns out, the default keyring identifier used by the Mailvelope plugin is in fact 'mailvelope'. Would it make more sense to default to that keyring instead of initializing a new one off of the user_id in Roundcube? Are there any conflicts that I'm not thinking of? I suggest this change since I wasn't even able to find the new roundcube keyring from the user interface in the first place.
To me this seems "more intuitive", but then again my HCI course isn't until the fall semester ;)
If this would be useful I would be more than happy to submit a PR.
Kyle