How do I equip an item to my main hand without opening my inventory?

I've seen it done on youtube, even in my own servers, where someone has an empty slot in their hot bar, and they press a key bind and like magic, the same item being used gets popped into the hot bar again. I would like to be able to finish using a stack of stone or something, and then just press a button and fill that hot bar slot again with another stack of stone. I can't find the answer anywhere and it would make my large projects take much less time.

2 Answers

It's not the same item again, it's the block they're looking at. If you for example have dirt somewhere in your inventory and press the "pick block" key (default is middle click) while looking at dirt, your currently selected item (preferably nothing) will be swapped with dirt.

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Mods, unless you're talking about from the off-hand to the main-hand, which is [F] by default, but from the way you worded your question, the answer is game mods (or server plugins).

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