If you have one bucket in your inventory and milk a cow with the "milk" button, the player will automatically start drinking the new milk bucket.

Tapping somewhere on the world will cancel the drinking (switching items will not), though I'm curious as to whether this might cause issues elsewhere with other items using similar functionality.

Edit: whoa. If you switch to another bucket on your hotbar from the milk bucket while in milking range of a cow, the bucket's contents will be replaced with milk and the player will start drinking from that bucket too.

Also, if one does not cancel the drink animation with a screen tap, switching to an Egg will throw it, and switching to a Bow will draw it but not fire. Switching from the bow to another item will not fire the arrow.