I had no problem with FIDO2 security key and other Webauth method so far.

Yes, I noticed that as well. And Google account as well, would not let me register a passkey from Firefox.

Brave, a.k.a Chrome with a lion logo…

You can try rclone but given the Proton Drive backen is in beta, it’s probably not going to be a simple setup.

I’ve never noticed the blanc lines until now, it never bothered me I guess. I believe Gmail behaves in the same way, also I haven’t use Gmail on mobile for a long while.