With the new Vector skin, I get an extra centimeter gap between the sidebar and everything else. I use iPadOS 13.5.1 and see this in both Safari and Firefox. It seems to be related to mw-content-container, as this closes the gap:

.mw-content-container { margin-left: 160px !important; }

But that, of course, disables use of the fullscreen mode and is no workaround for the problem.