It's a relatively obscure browser called Vivaldi, and the problem appeared over a range of versions (let's say 5.0 to today's 5.5). Vivaldi is a Chromium-based browser, and Chromium is an open-source browser on which Google Chrome is based as well, so you can think "approximately Google Chrome".And to top it up, my User-Agent is set to Firefox on Windows, because for some incomprehensible reason, there are websites that refuse to work if my browser doesn't pretend to be one of the major ones (and believe or not, I've also met websites that wouldn't work unless the User-Agent said that the browser ran on Windows! 
)
