A mod that doubles the size of the vanilla hotbar, from 9 slots to 18. It doesn't add any extra inventory slots, it just uses some that already exist in the player's inventory.
This mod is designed to only change what is absolutely required for functionality. At the time of writing all of the known crashes are a result of the other mods assuming that there are only 9 slots and not using the built-in Vanilla mechanism for finding the size of the hotbar.