1.21.9 Update Tracking · Issue #12563 · PaperMC/Paper – GitHub

Rework brigadier client tree building. Redirects make briagdier commands a graph instead of a tree. DFT is not a suitable algorithm for such data structures. To pick up a draggable item, press the space bar. While dragging, use the arrow keys to move the item. Press space again to drop the item in its new position, or press escape to cancel.

https://github.com/PaperMC/Paper/issues/12563 

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