Ensure effects are always cleaned up

.changeset/fix-unregister-bug.md

@@ -0,0 +1,5 @@
+---
+'@dnd-kit/abstract': patch
+---
+
+Make sure the cleanup function of effects is invoked when registering a new instance with the same `id` before the old instance has been unregistered.

packages/abstract/src/core/entities/entity/registry.ts

@@ -50,11 +50,18 @@ export class EntityRegistry<T extends Entity> {
    */
   public register = (key: UniqueIdentifier, value: T) => {
     const current = this.map.peek();
+    const currentValue = current.get(key);
 
-    if (current.get(key) === value) {
+    if (currentValue === value) {
       return;
     }
 
+    if (currentValue) {
+      const cleanup = this.cleanupFunctions.get(currentValue);
+      cleanup?.();
+      this.cleanupFunctions.delete(currentValue);
+    }
+
     const updatedMap = new Map(current);
     updatedMap.set(key, value);
 

packages/react/src/core/hooks/useInstance.ts

@@ -22,9 +22,8 @@ export function useInstance<T extends Instance>(
 
   useEffect(() => {
     instance.manager = manager;
-
-    // Register returns an unregister callback
-    return instance.register();
+    const unregister = instance.register();
+    return unregister;
   }, [instance, manager]);
 
   return instance;