fix(runtime-core): ensure consistent $options merge behavior with 2.x (#1986)

close #1978 , close #1979

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

@@ -457,7 +457,7 @@ describe('api: createApp', () => {
     app.config.optionMergeStrategies.foo = (a, b) => (a ? `${a},` : ``) + b
 
     app.mount(nodeOps.createElement('div'))
-    expect(merged!).toBe('global,extends,mixin,local')
+    expect(merged!).toBe('local,extends,mixin,global')
   })
 
   test('config.globalProperties', () => {

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

@@ -698,6 +698,50 @@ describe('api: options', () => {
     ])
   })
 
+  test('flatten merged options', async () => {
+    const MixinBase = {
+      msg1: 'base'
+    }
+    const ExtendsBase = {
+      msg2: 'base'
+    }
+    const Mixin = {
+      mixins: [MixinBase]
+    }
+    const Extends = {
+      extends: ExtendsBase
+    }
+    const Comp = defineComponent({
+      extends: defineComponent(Extends),
+      mixins: [defineComponent(Mixin)],
+      render() {
+        return `${this.$options.msg1},${this.$options.msg2}`
+      }
+    })
+
+    expect(renderToString(h(Comp))).toBe('base,base')
+  })
+
+  test('options defined in component have higher priority', async () => {
+    const Mixin = {
+      msg1: 'base'
+    }
+    const Extends = {
+      msg2: 'base'
+    }
+    const Comp = defineComponent({
+      msg1: 'local',
+      msg2: 'local',
+      extends: defineComponent(Extends),
+      mixins: [defineComponent(Mixin)],
+      render() {
+        return `${this.$options.msg1},${this.$options.msg2}`
+      }
+    })
+
+    expect(renderToString(h(Comp))).toBe('local,local')
+  })
+
   test('accessing setup() state from options', async () => {
     const Comp = defineComponent({
       setup() {