As you slide more items into the category, you can see a line forming that shows the extent of the respective category. To increase visibility, you can also enable “Change the background color of the items” and “Show horizontal bar over category” from Settings — Categories.

You can assign an item to a new category by dragging & dropping it into the new category, and remove it from the category by dragging & dropping it outside of the category. 

To collapse/ expand the category contents, tap on the visibility button (the ‘^’ sign at the left of the category name). In case the visibility button is not enabled, go to Settings — Categories — Show visibility button. Ta-daaa! No distractions on your way to the store if you only planned to buy that extra loaf of bread (and no ice-cream – but why would you even?!). 

Also, if you wish to expand/ collapse all categories at once, as a bulk action, go to the three dots menu, tap on Categories, and then select ‘All fold up’ or ‘All unfold’ from the sub-menu.