Why does the Vuetify navigation drawer fly out at large screen size? How do I prevent it from happening?

There is a prop to handle this exact behaviour disable-resize-watcher:

<v-navigation-drawer disable-resize-watcher v-model="sidebar" app>
</v-navigation-drawer>

https://vuetifyjs.com/en/components/navigation-drawers

Try those changes:

<v-navigation-drawer v-model="sidebar" app temporary>
<v-toolbar-side-icon @click="sidebar = !sidebar">

Also you can check the documentation about it here: https://vuetifyjs.com/en/components/navigation-drawers#props