message: Fix top nav to not stretch the full body, but only stretch the viewport width.
  This fixes popovers stretching the window out and ruining top nav.
diff: position: fixed
  top: 0
  left: 0
  - right: 0
  + // If we used `right: 0` instead, the top nav would flex off the screen if
  + // a horizontal scrollbar became visible. Doing it this way ensures it's
  + // always the correct width.
  + width: 100vw
  z-index: $zindex-shell-top-nav
  #shell.has-banner &
  