Add reproduction

Author: Weetbix

.gitignore

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+node_modules
+yarn-error.log

README.md

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-# vue-compat-composition-api-bug-repo
+# vue-compat-composition-api-bug-repo
+
+Reproduction for @vue/compat mode issue with composition API.
+
+`yarn test:vue3`
+
+`yarn test:vue-compat`

package.json

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+{
+  "name": "vue-compat-composition-api-bug-repo",
+  "version": "1.0.0",
+  "description": "",
+  "main": "index.js",
+  "scripts": {
+    "test:vue3": "vitest run",
+    "test:vue-compat": "vitest run --config vite.config.compat.js"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/Weetbix/vue-compat-composition-api-bug-repo.git"
+  },
+  "author": "",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/Weetbix/vue-compat-composition-api-bug-repo/issues"
+  },
+  "homepage": "https://github.com/Weetbix/vue-compat-composition-api-bug-repo#readme",
+  "dependencies": {
+    "@testing-library/vue": "^6.6.0",
+    "@vue/compat": "^3.2.37",
+    "vitest": "^0.17.1",
+    "vue": "^3.2.37"
+  },
+  "devDependencies": {
+    "jsdom": "^20.0.0"
+  }
+}

test.spec.js

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+
+
+import {ref, onMounted, defineComponent} from 'vue';
+import {it} from 'vitest';
+import {render} from '@testing-library/vue';
+
+function sleep(ms) {
+    return new Promise(resolve => setTimeout(resolve, ms));
+}
+
+const TestAsync = defineComponent({
+    template: '<div><div>{{ mountText }}</div><div>{{ asyncText }}</div></div>',
+
+    setup() {
+        const mountText = ref();
+        const asyncText = ref();
+
+        onMounted(() => {
+            mountText.value = 'mounted';
+        });
+
+        sleep(0).then(() => {
+            asyncText.value = 'async';
+        });
+
+        return {
+            mountText,
+            asyncText,
+        };
+    },
+});
+
+it('should show onMount text', async () => {
+    const {findByText} = render(TestAsync);
+    await findByText('mounted');
+});
+
+it('should show async text', async () => {
+    const {findByText} = render(TestAsync);
+    await findByText('async');
+});

vite.config.compat.js

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+import { defineConfig } from 'vite'
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+  },
+  resolve: {
+    alias: {
+      vue: '@vue/compat',
+    }
+  }
+})

vite.config.js

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+import { defineConfig } from 'vite'
+
+export default defineConfig({
+  test: {
+    environment: 'jsdom',
+  },
+})