fix: ensure undefined attributes are removed during hydration

[floriskn]

Attributes set server-side but undefined on the client should be removed during hydration. This ensures expected cleanup behavior and resolves a mismatch between SSR and client-side rendering.

Fixes sveltejs/svelte#16184

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 574def131957c4358e85917771405bfaef5b6ab9

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

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[svelte-docs-bot[bot]]

Preview: https://svelte-dev-git-preview-svelte-16178-svelte.vercel.app/

[7nik]

Please, add a test that fails without this fix. Propably it should go to the hydration folder, you can at the input-value-changed test as example of what you need.

[floriskn]

I’ve added the test under hydration named removes-undefined-attributes. It will fail without the fix, confirming the issue is properly covered.

[github-actions[bot]]

Playground

pnpm add https://pkg.pr.new/svelte@16178