fix: skip re-attaching shadow roots during view transitions

[Arecsu]

Changes

• Fixes: Elements with declarative Shadow DOM in transition-persisted containers throw "Unable to re-attach to existing ShadowDOM" error during view transitions
• The existing attachShadowRoots() function attempts to call attachShadow() on all elements with <template shadowrootmode>, but doesn't check if the element already has a shadow root from a previous page
• This commonly occurs when using transition:persist on containers with web components that use declarative Shadow DOM (e.g., custom elements, third-party libraries)
• The fix adds a simple check: if a shadow root already exists, skip re-attachment and just remove the template
• Follow-up of a recent PR made by @delucis: Attach declarative Shadow DOM templates during view transition #14341

Testing

This happened when fixed hydration mismatches in number-flow/svelte, which is a wrapper around a web component that uses Declarative Shadow DOM. PR: barvian/number-flow#162

The component was inside a transition-persisted flyout in all page-routes.

• Before: Navigation throws "Unable to re-attach to existing ShadowDOM" error
• After: Navigation works smoothly, shadow roots preserved correctly

Docs

Not applicable — bug fix for existing view transitions feature.

[changeset-bot]

🦋 Changeset detected

Latest commit: a712976

The changes in this PR will be included in the next version bump.

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[codspeed-hq]

CodSpeed Performance Report

Merging #14737 will not alter performance

_{Comparing Arecsu:main (a712976) with main (535fa36)¹}

Summary

✅ 6 untouched

Footnotes

1. No successful run was found on main (ab34340) during the generation of this report, so 535fa36 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Arecsu]

If this needs to have a changeset, please let me know. I'm not sure I fully understood if this PR needs it 😅

[delucis]

Looks good — thanks for the fix @Arecsu!

Yes, a changeset would be great here. This is a bug fix so it should be a patch to the astro package.

We have more guidance on how to write a changeset here: https://contribute.docs.astro.build/docs-for-code-changes/changesets/

[Arecsu]

Awesome. Done!

[delucis]

Perfect — nice work @Arecsu 🙌