fix: allow mutation of data and form props in Svelte 5

[dummdidumm]

By doing bind:data/form, the layout/child components are allowed to mutate those properties
Related sveltejs/svelte#9905 / sveltejs/svelte#10084

---

Please don't delete this checklist! 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
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

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

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: c7692a3

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

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	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

[benmccann]

I guess it makes sense for backwards compatibility. Is it what we want long-term? I'm not sure if we should remove the bind: in v3 or not

[dummdidumm]

We've officially encouraged this pattern in the past in order to do optimistic updates, so I don't think we can remove it in the future because that would mean you can't do that pattern anymore.

[benmccann]

lgtm, but probably wouldn't hurt to have rich take a look since I haven't kept up with svelte 5 as well

[Rich-Harris]

I'm torn on this. We definitely need some version of this, but in a world of proxied state it kinda feels like mutating data should cause $page.data to update elsewhere.

I think this pattern might be better:

-data.stuff = new_stuff;
+data = { ...data, stuff: new_stuff };

Slightly more verbose, but cleaner in my view. Is there any reason this wouldn't work?

[dummdidumm]

No reason this wouldn't work, just that we ourselves did data.stuff = .. in the past, and so people would have to refactor away from that to avoid the warnings they get with Svelte 5.