Sidebar [max-width: 413px] not working

For Desktop XL (1403px +)

The sidebar can grow obscenely wide on screens 1403+

The CSS prescribes a Sidebar max width of 413px but this is being ignored and it can grow much wider than this.!!

The site menu and widget area also breaks away from centre of sidebar to hug and even cross the sidebar right edge.

If there wasn’t a patch which adds more white behind the site menu it would be half in the sidebar and half in the gutter (at left of content).

The patch is:

body:before {
		width: -webkit-calc(50% - 289px);
		width: calc(50% - 289px);
	}

The rest of the media queries keeps the site menu centred within the sidebar.

I would like to work for the 1403+ viewport as well, but for this to look reasonable I need the max-width to hold.

Any ideas why max-width broken? How to fix?