Board Thread:Support Requests - Designing Your Wiki/@comment-35289850-20200817213219/@comment-45117243-20200818003240

Putting aside UCP, who knows what's going on there.

The content area is not % of the view, it has maximum of something like 1200px.

There are 2 types of background: the default is splitted bg, and I think this should fill the sides completly. The non-splitted type requires an image at least 2000px wide, and should take in account proportions so I guess that's why it does not fill the whole screen. The CSS style background-size:cover means to not care and do whatever is needed to cover the element, including streching or cropping.