Fix Bundles by simplifying them.

Let a Bundle become accessible when you hold it in the off-hand slot. That's all you should need to do. Forget about drag-and-drop, LIFO stacking, and so on.

Now when a Bundle is equipped in this way, it will just open up eight new inventory slots in your regular inventory GUI (outlined in red or something so you know they are only temporarily available and cannot exceed 64 items total).

Implementing Bundles in this way should make Bundles more friendly to handheld devices. All Inventory slots may need to be resized in order to make space for the new slots, but the choice of whether to shrink slots all the time or only when there's a Bundle open can be left up to the user.