SPIGOT-2000: Picking up items to shield slot working inconsistently when inventory is full

nms-patches/EntityItem.patch

@@ -127,6 +127,9 @@
 +
 +                // Possibly < 0; fix here so we do not have to modify code below
 +                this.pickupDelay = 0;
++            } else if (this.pickupDelay == 0) {
++                // ensure that the code below isn't triggered if canHold says we can't pick the items up
++                this.pickupDelay = -1;
 +            }
 +            // CraftBukkit end
 +

nms-patches/PlayerInventory.patch

@@ -85,6 +85,12 @@
 +            }
 +            if (remains <= 0) return itemstack.getCount();
 +        }
++        ItemStack offhandItemStack = this.getItem(this.items.size() + this.armor.size());
++        if (this.a(offhandItemStack, itemstack)) {
++            remains -= (offhandItemStack.getMaxStackSize() < this.getMaxStackSize() ? offhandItemStack.getMaxStackSize() : this.getMaxStackSize()) - offhandItemStack.getCount();
++        }
++        if (remains <= 0) return itemstack.getCount();
++
 +        return itemstack.getCount() - remains;
 +    }
 +    // CraftBukkit end

src/test/java/org/bukkit/craftbukkit/inventory/PlayerInventoryTest.java

@@ -56,5 +56,22 @@ public class PlayerInventoryTest extends AbstractTestingBase {
         assertEquals(1, inventory.canHold(itemStack1Coal));
         assertEquals(2, inventory.canHold(itemStack32Coal));
         assertEquals(2, inventory.canHold(itemStack64Coal));
+
+        // free space for 32 items in non-empty off-hand slot
+        inventory.setItem(inventory.items.size() - 1, itemStackApple);
+        inventory.setItem(inventory.items.size() - 2, itemStackApple);
+        inventory.setItem(inventory.items.size() + inventory.armor.size(), itemStack32Coal);
+
+        assertEquals(1, inventory.canHold(itemStack1Coal));
+        assertEquals(32, inventory.canHold(itemStack32Coal));
+        assertEquals(32, inventory.canHold(itemStack64Coal));
+
+        // free space for 1 item in non-empty off-hand slot and another slot
+        inventory.setItem(inventory.items.size() - 1, itemStack63Coal);
+        inventory.setItem(inventory.items.size() + inventory.armor.size(), itemStack63Coal);
+
+        assertEquals(1, inventory.canHold(itemStack1Coal));
+        assertEquals(2, inventory.canHold(itemStack32Coal));
+        assertEquals(2, inventory.canHold(itemStack64Coal));
     }
 }