If you start with a full hotbar of items, and you look straight at the ground, and start dropping them, and keep dropping them as they appear back into you inventory your inventory will start to fill up with whatever items you dropped. Here's the video I found on it. I just tested it in the 17w13b snapshot and it's still in this one as well.

https://www.youtube.com/watch?v=h0ecmAmZz7I