fix: detect and apply changes before or during initialisation

src/client/update.js

@@ -2,6 +2,13 @@
 let batchId = null
 
 export function triggerUpdate(vm, hookName) {
+  // if an update was triggered during initialization or when an update was triggered by the
+  // metaInfo watcher, set initialized to null
+  // then we keep falsy value but know we need to run a triggerUpdate after initialization
+  if (!vm.$root._vueMeta.initialized && (vm.$root._vueMeta.initializing || hookName === 'watcher')) {
+    vm.$root._vueMeta.initialized = null
+  }
+
   if (vm.$root._vueMeta.initialized && !vm.$root._vueMeta.paused) {
     // batch potential DOM updates to prevent extraneous re-rendering
     batchUpdate(() => vm.$meta().refresh())

src/shared/mixin.js

@@ -72,12 +72,29 @@ export default function createMixin(Vue, options) {
           this.$root._vueMeta.initialized = this.$isServer
 
           if (!this.$root._vueMeta.initialized) {
+            // we use the mounted hook here as on page load
             ensuredPush(this.$options, 'mounted', () => {
               if (!this.$root._vueMeta.initialized) {
+                // used in triggerUpdate to check if a change was triggered
+                // during initialization
+                this.$root._vueMeta.initializing = true
+
                 // refresh meta in nextTick so all child components have loaded
                 this.$nextTick(function () {
-                  const { metaInfo } = this.$root.$meta().refresh()
+                  const { tags, metaInfo } = this.$root.$meta().refresh()
+
+                  // After ssr hydration (identifier by tags === false) check
+                  // if initialized was set to null in triggerUpdate. That'd mean
+                  // that during initilazation changes where triggered which need
+                  // to be applied OR a metaInfo watcher was triggered before the
+                  // current hook was called
+                  // (during initialization all changes are blocked)
+                  if (tags === false && this.$root._vueMeta.initialized === null) {
+                    this.$nextTick(() => triggerUpdate(this, 'initializing'))
+                  }
+
                   this.$root._vueMeta.initialized = true
+                  delete this.$root._vueMeta.initializing
 
                   // add the navigation guards if they havent been added yet
                   // they are needed for the afterNavigation callback

test/unit/components.test.js

@@ -197,4 +197,80 @@ describe('client', () => {
     guards.after()
     expect(afterNavigation).toHaveBeenCalled()
   })
+
+  test('changes before hydration initialization trigger an update', async () => {
+    html.setAttribute(defaultOptions.ssrAttribute, 'true')
+
+    // this component uses a computed prop to simulate a non-synchronous
+    // metaInfo update like you would have with a Vuex mutation
+    const component = Vue.component('test-component', {
+      data() {
+        return {
+          hiddenTheme: 'light'
+        }
+      },
+      computed: {
+        theme() {
+          return this.hiddenTheme
+        }
+      },
+      beforeMount() {
+        this.hiddenTheme = 'dark'
+      },
+      render: h => h('div'),
+      metaInfo() {
+        return {
+          htmlAttrs: {
+            theme: this.theme
+          }
+        }
+      }
+    })
+
+    const wrapper = mount(component, { localVue: Vue })
+    expect(html.getAttribute('theme')).not.toBe('dark')
+
+    await vmTick(wrapper.vm)
+    jest.runAllTimers()
+
+    expect(html.getAttribute('theme')).toBe('dark')
+    html.removeAttribute('theme')
+  })
+
+  test('changes during hydration initialization trigger an update', async () => {
+    html.setAttribute(defaultOptions.ssrAttribute, 'true')
+
+    const component = Vue.component('test-component', {
+      data() {
+        return {
+          hiddenTheme: 'light'
+        }
+      },
+      computed: {
+        theme() {
+          return this.hiddenTheme
+        }
+      },
+      mounted() {
+        this.hiddenTheme = 'dark'
+      },
+      render: h => h('div'),
+      metaInfo() {
+        return {
+          htmlAttrs: {
+            theme: this.theme
+          }
+        }
+      }
+    })
+
+    const wrapper = mount(component, { localVue: Vue })
+    expect(html.getAttribute('theme')).not.toBe('dark')
+
+    await vmTick(wrapper.vm)
+    jest.runAllTimers()
+
+    expect(html.getAttribute('theme')).toBe('dark')
+    html.removeAttribute('theme')
+  })
 })