SPA + Prerendering: Problem with dynamic routes

[lennartkloock]

What I want to achieve

I'm trying to make a single page application which is partly prerendered. I want to have only static files after building.

The top +layout.svelte looks something like this:

// ...
// Always show top navigation bar
<TopNav />
{#if building}
    // Show loading spinner in prerender
    <LoadingSpinner />
{:else}
    // Show actual content when executed in browser
    <slot />
{/if}

+layout.ts

export const prerender = true;
export const ssr = true;

The problem

The application I'm developing has a dynamic page for each user. (/[username])
Technically it should be possible to show the prerendered layout for dynamic pages too since my prerendered output doesn't actually contain any dynamic content.

But I'm not sure how to configure Svelte to achieve this.

[lennartkloock]

Thinking about it and doing some research, I came to the conclusion that this is not possible with SvelteKit. Visiting a dynamic route means that the static file server always serves the fallback file. Which means, to modify how the response to the dynamic route looks, I would need to prerender the TopNav and LoadingSpinner component into the fallback file. Which is not possible with SvelteKit right now.