Sometimes, especially when using an external monitor attached to my laptop, the Chrome extension (used in Vivaldi in my case) displays its contents at the wrong scale. The Safari extension does the same thing. This is both the locked and unlocked screen. It doesn't respond to clicks in the expected way, but I can still use it if I stick to the keyboard.
Just checked and the menu bar app does the same thing too. However the desktop app looks fine.
Mac OS Mojave, latest Vivaldi, Safari, Enpass.