fix: reuse already attached attachments in spread attributes

[Ocean-OS]

Fixes #15949, needs a test before being merged

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: 2c45b7e

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@15951

[paoloricciuti]

I was fixing the same thing: this doesn't work because the effect created by attach is still destroyed by the parent template_effect...this means the attachment is detached and never reattached.

Imho we should have a separate $.attach_attachments function that creates an effect, reads only the symbols so it's only the attachments symbols so it's only triggered if any of those changes and then attach them

That would still trigger when any attachment change tho 🤔

So I think it's a bit annoying

[Ocean-OS]

I was fixing the same thing: this doesn't work because the effect created by attach is still destroyed by the parent template_effect...this means the attachment is detached and never reattached.

Imho we should have a separate $.attach_attachments function that creates an effect, reads only the symbols so it's only the attachments symbols so it's only triggered if any of those changes and then attach them

That would still trigger when any attachment change tho 🤔

So I think it's a bit annoying

I just realized that, so I'm looking into trying to make an attachment_effect that would make a derived from the attachment getter, and then only teardown if the derived has changed (or when any parent effects are permanently torn down), not when any parent effects have rerun.
I'm not sure how functional/feasible this would be, or if it's the best solution, but I'll try it out.
...And after looking into this more, it seems less and less feasible, so I'm going to try to find another way...

[paoloricciuti]

Mmm not clear if that would have any problem but I guess you can try

[Rich-Harris]

#15951

[Ocean-OS]

Since I'm fairly certain you meant to reference your new PR, I'll just tag it here
#15961

[Rich-Harris]

whoops! indeed