fix: maintain correct linked list of effects when updating each blocks

[Rich-Harris]

fixes #17184 — we now maintain the links between effects correctly (I think/hope) such that we never encounter an infinite loop

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]

🦋 Changeset detected

Latest commit: 216e9cc

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

This PR includes changesets to release 1 package

Name	Type
svelte	Patch

Not sure what this means? Click here to learn what changesets are.

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

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@17191

[dummdidumm]

brilliant!

[dummdidumm]

Just for clarification: This now keeps a link to the offscreen effects such that they are still connected precisely because prev.e.next is no longer set to null?

[Rich-Harris]

exactly — all effects are linked whether they're onscreen or offscreen, such that updates correctly flow through to offscreen effects as well as onscreen ones