Seems like when I offer too little and get "A bit more", it does not allow me to try another transaction without quitting the trade entirely. Deselecting and reselecting the item does not work, changing to a different one and back, nope. Cannot trigger "Hmm" again during that session.
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Do you have more specific instructions for replicating this? It seems to work for me.
It could be related to the code that stops the NPC from saying "Hmm" until you stop moving your cursor around. I might add a hard upper limit where the NPC will eventually evaluate your offer even if you're moving the cursor.