And the changes start coming and they don't stop coming

eco-api/src/main/java/com/willfp/eco/core/gui/slot/FillerSlot.java

@@ -30,4 +30,9 @@ public class FillerSlot implements Slot {
     public ItemStack getItemStack(@NotNull final Player player) {
         return itemStack;
     }
+
+    @Override
+    public boolean isCaptive() {
+        return false;
+    }
 }

eco-api/src/main/java/com/willfp/eco/core/gui/slot/Slot.java

@@ -19,6 +19,13 @@ public interface Slot {
      */
     ItemStack getItemStack(@NotNull Player player);
 
+    /**
+     * If the slot is captive. (Can items be placed in it).
+     *
+     * @return If captive.
+     */
+    boolean isCaptive();
+
     /**
      * Create a builder for an ItemStack.
      *