[Bug]: Decorators in Svelte are executed twice

[JReinhold]

Describe the bug

Decorator in Svelte are always executed twice during story rendering. However Svelte components in decorators are (correctly) only mounted once.

To Reproduce

1. Open https://stackblitz.com/edit/sveltejs-kit-template-default-ys5aaj?file=.storybook%2Fpreview.ts (SvelteKit sandbox)
2. npm run storybook
3. Navigate to a Button story
4. Open the Console
5. See that in the logs, "log in decorator" is logged twice, while "TestDecorator onMount" and "Button onMount" are logged once.

System

Storybook Environment Info:

  System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 18.18.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 9.4.2 - /usr/local/bin/npm <----- active
    pnpm: 8.9.2 - /usr/local/bin/pnpm
  npmPackages:
    @storybook/addon-essentials: ^7.5.1 => 7.5.1 
    @storybook/addon-interactions: ^7.5.1 => 7.5.1 
    @storybook/addon-links: ^7.5.1 => 7.5.1 
    @storybook/blocks: ^7.5.1 => 7.5.1 
    @storybook/svelte: ^7.5.1 => 7.5.1 
    @storybook/sveltekit: ^7.5.1 => 7.5.1 
    @storybook/testing-library: ^0.2.2 => 0.2.2 
    storybook: ^7.5.1 => 7.5.1

Additional context

A simple workaround is to move any logic in the decorator that is not supposed to run twice, from the decorator into a Svelte component's onMount.

Before:

// preview.js

...
decorators: [
	(Story) => {
        // some code that cannot run twice
		return Story();
	},
]
...

After:

// preview.js
import MyDecorator from './MyDecorator.svelte';

...
decorators: [
	() => MyDecorator,
]
...

// MyDecorator.svelte
<script>
  import { onMount } from 'svelte';

  onMount(() => {
    // some code that cannot run twice
  })
</script>

<slot/>