Out of habit (Baldur’s Gate and Pillars of Eternity, anyone), I dragged weapons and armour onto the portraits of battle brothers in the inventory. When you do that, it seems to work – the portrait gets highlighted, you get a sound – and then of course nothing happens. The item seemingly vanishes, but in fact just gets put on a field way down in your inventory.
I’d suggest dragging items onto brothers either puts the dragged item into the respective personal inventory slot (weapon into weapon slot) to replace the currently equipped item or doesn’t do anything at all, just making the item reappear in the field that you dragged it from.
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