Board Thread:Support Requests - Designing Your Wiki/@comment-32299625-20180628173317/@comment-9605025-20180630211936

I understand now. Unfortunately, you cannot perfectly align the image since the extra space on the sides is not necessarily the same every time someone views the page. The amount of extra space on the sides depends on the size of the window the user is using to view the page. True, since we used "background-size: cover;", the size of the background is also scaled to the window size. However, the page content is only pseudo-scaled. Wikia groups windows into, I think, 4 size classes and provides a fixed content size for each group. This means that, within a group, the proportion of the window taken by the main content varies. Since you cannot separately scale different parts of a single image, it is therefore not possible to get the edges in your composite image to line-up with the edge of the page content.