How can FileViewer be messing with XenForo's software, your web site, and my browser?
Before recent forum upgrades, <code> blocks appeared monospaced by default. I have probably posted hundreds of code blocks and I never, until recently, had to change the font manually (a P.I.T.A.). I'm pretty sure code blocks from weeks/months/years ago are monospaced.
Directly - no. But through a vehement complaint to XenForo about a giant step backward - you may be able to, or at least influence it. Since only you have a contract with XenForo, neither Vince, nor I, nor any other of your customers can get this malfeature removed. After all, what is the point of the <code> tag if it does not create a visually different format?
Again, I have no idea. You must see it too because I didn't mention the pop-up. The pop-up is for "V", and given that you have some relationship with "V", it seems fishy. I'm so curious! Will you get to the bottom of this mystery (and put an end to it)?
It sounds like you have contacted XenForo. Did they change the default font for code blocks and leave the user (meaning you) no way to change it?
So I asked about it in XenForo's general forum and got a quick answer. It's completing forum member usernames (and XenForo didn't know that!). It offers to complete "@vef". I asked if the site's admin could turn it off and also if the admin could set a default font for code blocks.