Hi, this is happening because the template created by create-next-app contains:
<main className="min-h-screen flex flex-col items-center">
in RootLayout.tsx, you can have remove min-h-screen or disable the layout for the PlasmicHost page, this should fix the issue of your canvas elements not positioned properly.
About the code components, could you double check your implementation/registration? The file where registerComponent should be called is different in the app dir setup.