Vue 3 @click methods $options.x is not a function

Totally stumped on this. I’m developing a WordPress plugin with Vue but I can’t get the @click event to work.

<script>
export default {
  name: "App",

  methods: {
    showPanel: function () {
      console.log("Clicked!");
    }
  }
};
</script>

<template>
  <div id="frontend-app" class="app">
    <button @click="showPanel()">Test Button</button>
    <!-- <router-view /> -->
  </div>
</template>

But I get the following warning and error:

runtime-core.esm-bundler.js:179  [Vue warn]: Unhandled error during execution of native event handler 
  at <App>

Uncaught TypeError: $options.showPanel is not a function
    at onClick._cache.<computed>._cache.<computed> (App.vue?./node_modules/babel-loader/lib/index.js!./node_modules/vue-loader/dist/templateLoader.js??ruleSet%5B1%5D.rules%5B2%5D!./node_modules/vue-loader/dist/index.js??ruleSet%5B1%5D.rules%5B8%5D.use%5B0%5D:13:59)
    at callWithErrorHandling (runtime-core.esm-bundler.js:296:18)
    at callWithAsyncErrorHandling (runtime-core.esm-bundler.js:304:17)
    at HTMLButtonElement.invoker (runtime-dom.esm-bundler.js:437:82)

I used Vue.js 3 Plugin Generator to setup the WordPress plugin if that’s relevant and the Vue app is built with Webpack.

Any ideas? I’ve searched all over and tried everything I can think of.

Thanks