Category selector doesn't work properly at various resolutions

[russkev]

At the moment, the site thinks that 'food' should be selected even if there is just a hint of the food card being on screen. This means that if the resolution is such that even when scrolled all the way to the left, a food card is still visible, it will not let the drinks menu item appear highlighted, effectively breaking the selector.

The solution is to have it so that the food item has to be at least half way across the visible area to trigger the menu to be in the 'food' selection mode.