The 1.21.5 branch has now been merged, thanks to all who tested it!
Inventory event processing has been reworked somewhat in this version. The InventoryCondition API has been removed entirely in favor of InventoryPreClickEvent.
Click information in InventoryPreClickEvent has been condensed into the new Click union. It is suggested to structure these checks using switch statements, eg:
switch (event.getClick()) {