fix: defer effects in pending boundary

[Rich-Harris]

just a failing test for now. #16738 (comment)

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• Prefix your PR title with feat:, fix:, chore:, or docs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code within packages/svelte/src, add a changeset (npx changeset).

Tests and linting

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

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8bb6adf

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@16743

[dummdidumm]

#16971 is merged and contains one of the tests in here but no the other one - not sure what you wanna do with it

[Rich-Harris]

gonna keep it around for now, until we can come up with a fix. it's not urgent though IMHO — it handles something that's a bit of an edge case that most people will never notice

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-16743-svelte.vercel.app/

[Rich-Harris]

got this to work (okay, okay... opencode got this to work) but might try and tidy it up a little more before marking it ready