PaperMC/paper-server/nms-patches/net/minecraft/world/level/block/BlockDropper.patch

--- a/net/minecraft/world/level/block/BlockDropper.java
+++ b/net/minecraft/world/level/block/BlockDropper.java
@@ -15,6 +15,12 @@
 import net.minecraft.world.level.block.state.BlockBase;
 import net.minecraft.world.level.block.state.IBlockData;
 
+// CraftBukkit start
+import net.minecraft.world.InventoryLargeChest;
+import org.bukkit.craftbukkit.inventory.CraftItemStack;
+import org.bukkit.event.inventory.InventoryMoveItemEvent;
+// CraftBukkit end
+
 public class BlockDropper extends BlockDispenser {
 
     private static final IDispenseBehavior DISPENSE_BEHAVIOUR = new DispenseBehaviorItem();
@@ -52,8 +58,25 @@
                 if (iinventory == null) {
                     itemstack1 = BlockDropper.DISPENSE_BEHAVIOUR.dispense(sourceblock, itemstack);
                 } else {
-                    itemstack1 = TileEntityHopper.addItem(tileentitydispenser, iinventory, itemstack.copy().split(1), enumdirection.getOpposite());
-                    if (itemstack1.isEmpty()) {
+                    // CraftBukkit start - Fire event when pushing items into other inventories
+                    CraftItemStack oitemstack = CraftItemStack.asCraftMirror(itemstack.copy().split(1));
+
+                    org.bukkit.inventory.Inventory destinationInventory;
+                    // Have to special case large chests as they work oddly
+                    if (iinventory instanceof InventoryLargeChest) {
+                        destinationInventory = new org.bukkit.craftbukkit.inventory.CraftInventoryDoubleChest((InventoryLargeChest) iinventory);
+                    } else {
+                        destinationInventory = iinventory.getOwner().getInventory();
+                    }
+
+                    InventoryMoveItemEvent event = new InventoryMoveItemEvent(tileentitydispenser.getOwner().getInventory(), oitemstack.clone(), destinationInventory, true);
+                    worldserver.getCraftServer().getPluginManager().callEvent(event);
+                    if (event.isCancelled()) {
+                        return;
+                    }
+                    itemstack1 = TileEntityHopper.addItem(tileentitydispenser, iinventory, CraftItemStack.asNMSCopy(event.getItem()), enumdirection.getOpposite());
+                    if (event.getItem().equals(oitemstack) && itemstack1.isEmpty()) {
+                        // CraftBukkit end
                         itemstack1 = itemstack.copy();
                         itemstack1.shrink(1);
                     } else {