In 1.16.5, when you try to control drop a slot while having a creative GUI opened (doesn't apply when opening the inventory screen of creative), it doesn't work. It's like if the item just disapeared.
As being a modder, this is more a Server Side problem, since the client only sends a packet. (This is what I'm understanding, but I could be entirely wrong).
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Duplicate ofย MC-170863.

True, oops. I guess this ticket can be closed then !

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โ since the client only sends a packet
I would hope you didn't expect the client to send 64 packets when you do this; it could be fixed by just sending a different/changed packet.