Moving through tiers on mobile nav improved

When scrolling through tiers of navigation on the mobile nav, the browser window now 'locks' the viewport at a scroll height, make it easier to navigate the tiered menu.

Before, if you moved through a tier of navigation which a lot of links, you'd have to scroll down, then selecting a child with fewer, the updated child tier would be displayed above and outside the viewport, leading to the user having to scroll back up just to continue browsing the menu tree, an inconsistent experience.

Now, each scrolling across to the child level action, also resets the scroll to the top of the expanded drop down, making the moving between tiers consistent and easier on a small device.