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fix: prevent component pre-mounting in async boundaries #16633

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@harshmandan harshmandan commented Aug 16, 2025
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Note: Follows up #16624.

failing test case w/ 16624 | The same test case with this PR


Description:
This PR changes how deferred components and their effects are mounted. Earlier components inside svelte:boundary with await were being mounted immediately as offscreen fragments and then replaced/swapped after the promises resolved, triggering effects immediately.

In this PR mounting is deferred until promise is resolved (no offscreen fragment is created. Not sure if this has any implications 🤞).

So, this solves the original issue of changed lifecycle order and the component unmounting is also kept deferred.

Tests and linting

  • Run the tests with pnpm test and lint the project with pnpm lint

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changeset-bot bot commented Aug 16, 2025
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🦋 Changeset detected

Latest commit: eebadcd

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

This PR includes changesets to release 1 package
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svelte Minor

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Playground

pnpm add https://pkg.pr.new/svelte@16633

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harshmandan commented Aug 16, 2025
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The test passes locally. Not sure why the CI run failed.

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@harshmandan harshmandan force-pushed the harsh/fix/prevent-component-pre-mounting branch from 5fb1ab6 to 1ac643f Compare August 16, 2025 18:42
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