Fix RouterView instance registration under HMR (Options API beforeRouteUpdate)

packages/router/__tests__/RouterView.hmr.spec.ts

@@ -0,0 +1,124 @@
+/**
+ * @vitest-environment jsdom
+ */
+import { describe, it, expect, vi } from 'vitest'
+import type { WatchStopHandle } from 'vue'
+
+describe('RouterView – HMR (Options API)', () => {
+  it('Registers the instance on re-mount → nextTick via onVnodeMounted, even if watch(post) is blocked', async () => {
+    // 1) Reset modules and prepare hoisted state to track blocked post-watch registrations
+    vi.resetModules()
+    const hoisted = vi.hoisted(() => ({ blockedPostWatchCalls: 0 }))
+
+    // 2) Mock `vue`: block only watch({ flush: "post" }), delegate all others to the real implementation
+    vi.doMock('vue', async () => {
+      const actual = await vi.importActual<typeof import('vue')>('vue')
+      return {
+        ...actual,
+        watch: (source: any, cb: any, options: any) => {
+          if (options?.flush === 'post') {
+            hoisted.blockedPostWatchCalls++
+            return (() => {}) as WatchStopHandle
+          }
+          return (actual as any).watch(source, cb, options)
+        },
+      }
+    })
+
+    // 3) Dynamic import (after mocking is applied!)
+    const { defineComponent, h, ref, nextTick } = await import('vue')
+    const { mount } = await import('@vue/test-utils')
+
+    // 4) Import router/RouterView and enable HMR flag
+    ;(globalThis as any).__DEV__ = true
+    ;(import.meta as any).hot = {}
+    const { createRouter, createMemoryHistory, RouterView } = await import(
+      '../src'
+    )
+
+    // 5) Define test component & router
+    const beforeUpdateSpy = vi.fn()
+    const OptComp = {
+      template: `<div>Opt</div>`,
+      beforeRouteUpdate(to: any) {
+        beforeUpdateSpy(to.params.id)
+      },
+    }
+
+    const router = createRouter({
+      history: createMemoryHistory(),
+      routes: [{ path: '/temp/:id', components: { default: OptComp } }],
+    })
+
+    const rvKey = ref(0)
+    const App = defineComponent({
+      setup() {
+        return () => h('div', [h(RouterView, { key: rvKey.value })])
+      },
+    })
+
+    mount(App, { global: { plugins: [router] }, attachTo: document.body })
+    await router.push('/temp/1')
+    await router.isReady()
+
+    // 6) Prepare to track writes to rec.instances.default
+    const rec = router.currentRoute.value.matched[0]
+    expect(rec).toBeTruthy()
+
+    if (rec?.instances && 'default' in rec.instances) {
+      // @ts-ignore
+      rec.instances.default = null
+    }
+
+    const desc = Object.getOwnPropertyDescriptor(rec.instances, 'default')
+    let stored: any
+    let preWrites = 0
+    let postWrites = 0
+    let phase: 'pre' | 'post' = 'pre'
+
+    Object.defineProperty(rec.instances, 'default', {
+      configurable: true,
+      get() {
+        return stored
+      },
+      set(v) {
+        stored = v
+        if (phase === 'pre') {
+          preWrites++
+        } else {
+          postWrites++
+        }
+      },
+    })
+
+    // 7) Re-mount → onVnodeMounted hasn’t fired yet in the same tick
+    rvKey.value++
+    expect(preWrites).toBe(0)
+
+    // 8) Next tick: onVnodeMounted should register the instance (post-watch is blocked)
+    await nextTick()
+    expect(preWrites).toBeGreaterThanOrEqual(1) // recorded via pre-registration only
+    expect(Boolean(stored)).toBe(true)
+
+    // 9) Navigation → post-watch is blocked, so postWrites must remain 0
+    phase = 'post'
+    const navDone = new Promise<void>(resolve => {
+      const remove = router.afterEach(() => {
+        remove()
+        resolve()
+      })
+    })
+    router.push('/temp/2')
+    await navDone
+
+    expect(postWrites).toBe(0) // no post-watch writes
+    expect(hoisted.blockedPostWatchCalls).toBeGreaterThanOrEqual(1) // attempted to register post-watch
+    expect(beforeUpdateSpy).toHaveBeenCalledWith('2') // hook still works
+
+    // 10) Restore original property & unmock
+    if (desc) Object.defineProperty(rec.instances, 'default', desc)
+    else delete (rec.instances as any).default
+    ;(rec.instances as any).default = stored
+    vi.doUnmock('vue')
+  })
+})

packages/router/src/RouterView.ts

@@ -165,12 +165,36 @@ export const RouterViewImpl = /*#__PURE__*/ defineComponent({
         }
       }
 
+      const isHMRRuntime =
+        (__DEV__ || __FEATURE_PROD_DEVTOOLS__) &&
+        isBrowser &&
+        (import.meta as any).hot
+
+      const onVnodeMounted: VNodeProps['onVnodeMounted'] | undefined =
+        isHMRRuntime
+          ? (vnode: VNode) => {
+              if (
+                matchedRoute &&
+                vnode.component &&
+                matchedRoute === route.matched[depth.value]
+              ) {
+                matchedRoute.instances[currentName] = vnode.component.proxy
+              }
+            }
+          : undefined
+
       const component = h(
         ViewComponent,
-        assign({}, routeProps, attrs, {
-          onVnodeUnmounted,
-          ref: viewRef,
-        })
+        assign(
+          {},
+          routeProps,
+          attrs,
+          onVnodeMounted ? { onVnodeMounted } : null,
+          {
+            onVnodeUnmounted,
+            ref: viewRef,
+          }
+        )
       )
 
       if (