test: add test case

Author: edison1105

packages/reactivity/src/effectScope.ts

@@ -78,7 +78,7 @@ export class EffectScope {
     if (this.active) {
       let i, l
       // #5783
-      // effects maybe changed when an effect stoped.
+      // effects will be changed when a watcher stoped.
       // so we need to copy it for iteration.
       const effectsToStop = this.effects.slice()
       for (i = 0, l = effectsToStop.length; i < l; i++) {

packages/runtime-core/__tests__/apiWatch.spec.ts

@@ -1118,4 +1118,26 @@ describe('api: watch', () => {
     // own update effect
     expect(instance!.scope.effects.length).toBe(1)
   })
+
+  test('handle nested watcher stop properly', () => {
+    let instance: ComponentInternalInstance
+    const Comp = {
+      setup() {
+        instance = getCurrentInstance()!
+        watch(
+          () => 1,
+          (val, oldVal, onCleanup) => {
+            const stop = watch(() => 2, () => { })
+            onCleanup(stop)
+          },
+          { immediate: true }
+        )
+        return () => ''
+      }
+    }
+    const root = nodeOps.createElement('div')
+    createApp(Comp).mount(root)
+    expect(instance!.scope.effects.length).toBe(3)
+    instance!.scope.stop()
+  })
 })