Topic: 8 Million Item Stack Max?
Maximum item qty stack is somewhere between 8-9 million, and exceeding this will delete any beyond it. It's rare that players deal with quantities this high, but it's not out of the realm of possibly very high gold transactions that would ruin people's day and result in confusion & suspicion.
From what I can tell, 7 million is the maximum amount of any item stack that can be moved. So, if you have 8 million in inventory and drop all, it will only drop 7 million. Then, you can drop the remaining 1 million. Both stacks can be picked up, but if you try to pick up another 1 million for a total of 9, the added amount is deleted (still leaving the 8 million only). Poof, gone.
Another test- If you put the 8 million into a bank slot, it will only allow moving 7 million (the same limit as dropping). You can still move the next 1 million, for a total of 8million in a bank slot. But, when you add 1 more million, it deletes a portion of it. You end up with approximately 8.5million (or some odd amount I don't recall). Moving another 100 on top of that seemed to work without deleting. From here, I imagine it just slowly reveals the true item stack maximum (between 8-9 million).