PaperMC/paper-server/patches/sources/net/minecraft/world/CompoundContainer.java.patch

--- a/net/minecraft/world/CompoundContainer.java
+++ b/net/minecraft/world/CompoundContainer.java
@@ -7,6 +_,48 @@
     public final Container container1;
     public final Container container2;
 
+    // Paper start - add fields and methods
+    public java.util.List<org.bukkit.entity.HumanEntity> transaction = new java.util.ArrayList<org.bukkit.entity.HumanEntity>();
+
+    public java.util.List<ItemStack> getContents() {
+        java.util.List<ItemStack> result = new java.util.ArrayList<>(this.getContainerSize());
+        for (int i = 0; i < this.getContainerSize(); i++) {
+            result.add(this.getItem(i));
+        }
+        return result;
+    }
+
+    public void onOpen(org.bukkit.craftbukkit.entity.CraftHumanEntity who) {
+        this.container1.onOpen(who);
+        this.container2.onOpen(who);
+        this.transaction.add(who);
+    }
+
+    public void onClose(org.bukkit.craftbukkit.entity.CraftHumanEntity who) {
+        this.container1.onClose(who);
+        this.container2.onClose(who);
+        this.transaction.remove(who);
+    }
+
+    public java.util.List<org.bukkit.entity.HumanEntity> getViewers() {
+        return this.transaction;
+    }
+
+    public org.bukkit.inventory.InventoryHolder getOwner() {
+        return null; // This method won't be called since CraftInventoryDoubleChest doesn't defer to here
+    }
+
+    public void setMaxStackSize(int size) {
+        this.container1.setMaxStackSize(size);
+        this.container2.setMaxStackSize(size);
+    }
+
+    @Override
+    public org.bukkit.Location getLocation() {
+        return this.container1.getLocation(); // TODO: right?
+    }
+    // Paper end
+
     public CompoundContainer(Container container1, Container container2) {
         this.container1 = container1;
         this.container2 = container2;
@@ -58,7 +_,7 @@
 
     @Override
     public int getMaxStackSize() {
-        return this.container1.getMaxStackSize();
+        return Math.min(this.container1.getMaxStackSize(), this.container2.getMaxStackSize()); // Paper - check both sides
     }
 
     @Override