Whenever I hold down middle click (scroll wheel) and drag it around my inventory, any slot that it touches, becomes full of a stack of that item. It's just visual, so when you click again, or exit the inventory it disappears, so no duplication glitches here, it's purely visual.
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But the problem is, in survival mode, when you drag the cursor, it duplicates the items, creating stacks in each slot you drag it over. I'm probably just bad at explaining things, but I doubt that's supposed to happen.
See MC-46584
That's what the middle mouse button is supposed to do. See https://minecraft.gamepedia.com/Inventory for more info about inventory management.