FOUC occurs on SSR when deployed to vercel and dynamically importing a component

[mattjennings]

Describe the bug
When dynamically importing a component through load and passing it to the page through props, there is a flicker of unstyled content when that page is SSR'd.

Logs
no logs to show

To Reproduce

Repo: https://github.com/mattjennings/sveltekit-vercel-FOUC
Live URL: https://svelte-bug.vercel.app

<!-- $lib/Button.svelte -->
<button class="button">button</button>

<style>
  .button {
    background: blue;
  }
</style>

<!-- routes/index.svelte -->
<script context="module">
  export async function load() {
    const Button = await import("$lib/Button.svelte");

    return {
      props: {
        component: Button.default,
      },
    };
  }
</script>

<script>
  export let component;
</script>

<svelte:component this={component} />

Deploy to vercel, load the page SSR and the FOUC will occur. It does not happen when running in dev, previewing locally, or when client-side navigating to the page on the deployment.

Expected behavior
FOUC should not occur

Information about your SvelteKit Installation:

Diagnostics

System: OS: macOS 11.4 CPU: (16) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz Memory: 231.64 MB / 16.00 GB Shell: 5.8 - /bin/zsh Binaries: Node: 16.3.0 - ~/.nvm/versions/node/v16.3.0/bin/node npm: 7.15.1 - ~/.nvm/versions/node/v16.3.0/bin/npm Browsers: Chrome: 91.0.4472.114 Safari: 14.1.1 npmPackages: @sveltejs/adapter-vercel: ^1.0.0-next.24 => 1.0.0-next.24 @sveltejs/kit: next => 1.0.0-next.123 svelte: ^3.34.0 => 3.38.3

Severity
How severe an issue is this bug to you? Is this annoying, blocking some users, blocking an upgrade or blocking your usage of SvelteKit entirely?

Annoying

Additional context
I'm not sure how long this has been happening. I have an old deployment of an affected project using @sveltejs/[email protected] and @sveltejs/[email protected] that it does not happen in. However, a new project with those versions still has the problem.

[benmccann]

Sounds like a duplicate of #967