Cover size?

We have standardised™ and regulated sizes for avatars and signatures and while I'd usually want to criticise those for being too restrictive, today I come with a questions. Cover photos seem to have no standard, neither within the community nor among the moderators and I guess not even the platform itself really sets one? The height seems fixed to 200 px, but the width appears to be limited only by the width of the browser window, which in turn is restricted through hardware, software and the explicit resizing operations done by the user. On my humble display, I can not even hope to grasp this infinity with an upper limit cover width of mere 1318 px. For those wondering, yes, that's QHD, please don't judge. As I'd like to share art™ with my comrades, I wanted to ask other users for their limitations (both cover and screen). Also, if the staff could (in)validate my claim about limitations set by the platform, that'd be great.

The full image in all of its glory or again just the white part?

So just to make sure: PCs show the white part and mobile phones as well as other browsers disguising as such show the full image. Correct?

Cool. Now that I've gained this information, I finally have a deep understanding of the system's inner workings. My questions have been answered.