If you perform an action after the drop quantity option appears on screen it often time freezes on screen, you can move the window but the x button and the drop button don't work. It comes about in various ways, say you're dropping an item, say you forget to hit the drop button when it asks you to verify the quantity while clearing your inventory and go to drop another item it will trigger it*sometimes*. Here's a good way for you to test it Zer: Open a trade window with another player, drop something stackable on the ground but dont hit the drop or x button, just leave the window up. Then trade the item to the other player. This triggers the state I'm describing most of the time.
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