Client:only Svelte 4 or 5 component does not remove fallback slot after it loaded

[zenshindm]

Astro Info

Astro                    v4.16.5
Node                     v20.15.0
System                   Windows (x64)
Package Manager          npm
Output                   static
Adapter                  none
Integrations             @astrojs/mdx
                         @astrojs/sitemap
                         @astrojs/tailwind
                         @astrojs/svelte
                         @astrojs/partytown

If this issue only occurs in one browser, which browser is a problem?

Chrome

Describe the Bug

https://docs.astro.build/en/reference/directives-reference/#clientonly

according to the docs, you can do fallback slot with client only svelte component

Loading

but the after the component loaded , the fallback "loading" still there

i have test on svelte 4 and 5 , it does not work
but it work on react
see the codesandbox for example

What's the expected result?

i expected the fallback gone after Client Component loaded

Link to Minimal Reproducible Example

https://codesandbox.io/p/devbox/5ts6ky?file=%2Fsrc%2Fcomponents%2FButton.jsx%3A4%2C5

Participation

• I am willing to submit a pull request for this issue.

[haltcase]

I'm seeing this too with a Svelte 5 component. At first I thought it might be because Svelte also uses <slot /> (though deprecated in v5), but I can't reproduce with a Vue component (which also supports <slot />).

[dually8]

I'm seeing the same. I've been working in the Astro blog template. I have a toggle button written in Svelte 5 and the fallback isn't being removed on load.

[dominiwe]

I have observed the same issue. Seems this is currently broken entirely for Svelte.