fix(runtime-core): align $parent/$root with the template ref when using expose (#3158)

packages/runtime-core/src/componentPublicInstance.ts

@@ -1,4 +1,8 @@
-import { ComponentInternalInstance, Data } from './component'
+import {
+  ComponentInternalInstance,
+  Data,
+  isStatefulComponent
+} from './component'
 import { nextTick, queueJob } from './scheduler'
 import { instanceWatch, WatchOptions, WatchStopHandle } from './apiWatch'
 import {
@@ -207,8 +211,11 @@ type PublicPropertiesMap = Record<string, (i: ComponentInternalInstance) => any>
  */
 const getPublicInstance = (
   i: ComponentInternalInstance | null
-): ComponentPublicInstance | null =>
-  i && (i.proxy ? i.proxy : getPublicInstance(i.parent))
+): ComponentPublicInstance | ComponentInternalInstance['exposed'] | null => {
+  if (!i) return null
+  if (isStatefulComponent(i)) return i.exposed ? i.exposed : i.proxy
+  return getPublicInstance(i.parent)
+}
 
 const publicPropertiesMap: PublicPropertiesMap = extend(Object.create(null), {
   $: i => i,
@@ -219,7 +226,7 @@ const publicPropertiesMap: PublicPropertiesMap = extend(Object.create(null), {
   $slots: i => (__DEV__ ? shallowReadonly(i.slots) : i.slots),
   $refs: i => (__DEV__ ? shallowReadonly(i.refs) : i.refs),
   $parent: i => getPublicInstance(i.parent),
-  $root: i => i.root && i.root.proxy,
+  $root: i => getPublicInstance(i.root),
   $emit: i => i.emit,
   $options: i => (__FEATURE_OPTIONS_API__ ? resolveMergedOptions(i) : i.type),
   $forceUpdate: i => () => queueJob(i.update),