Attachment using `createAttachmentKey` re-runs when other spread state changes

[CaptainCodeman]

Describe the bug

Looks like a similar issue to #15949 but if other spread props are updated the attachment still re-runs.

I would expect to be able to attach behaviour to an element using createAttachmentKey while also setting attributes on the element (such as wai-aria).

Reproduction

https://svelte.dev/playground/542b5db158e148feafde1451c6cbd900?version=5.33.10

(click button, notice console log output showing attachment re-runs)

Logs

System Info

System:
    OS: macOS 15.5
    CPU: (10) arm64 Apple M1 Pro
    Memory: 1.68 GB / 16.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.12.0 - ~/Library/pnpm/node
    npm: 11.1.0 - ~/Library/pnpm/npm
    pnpm: 10.11.0 - ~/Library/pnpm/pnpm
  Browsers:
    Chrome: 136.0.7103.116
    Safari: 18.5
  npmPackages:
    svelte: ^5.0.0 => 5.33.10

Severity

blocking an upgrade

[7nik]

You create a new function in the derived, so it definitely will re-attach. But the issue is real.

[PatrickG]

@7nik You create a new attachment key in the derived.
Works.

[7nik]

Then it should be mentioned in the docs

[CaptainCodeman]

or could createAttachmentKey possibly return the same key based on the scope it's in?

[7nik]

You may want to create multiple attachments.

But talking about not re-attaching fn even the key changes, wouldn't it be more convenient?

[CaptainCodeman]

it seems you need both the key and the fn body to be external to the $derived, so I'll probably go with something like this:

const buttonAttachment = {
	[createAttachmentKey()]: (node: HTMLElement) => {
		console.log('createButtonAttachment', faq.id)
		buttonNode = node;

		on(node, 'click', () => expanded = !expanded)
		on(node, 'focus', () => node.classList.add('focus'))
		on(node, 'blur', () => node.classList.remove('focus'))
	}
}

const button = $derived({
	type: 'button' as const,
	'aria-expanded': expanded,
	'aria-controls': `faq-${faq.id}`,
	...buttonAttachment,
})

[paoloricciuti]

You may want to create multiple attachments.

But talking about not re-attaching fn even the key changes, wouldn't it be more convenient?

Mh i don't know if that would work but I think is worth exploring.

@Conduitry will reopen this since I think documenting this could still be worth it, I'll try to see if it's fixable, if it's not will document

[paoloricciuti]

But talking about not re-attaching fn even the key changes, wouldn't it be more convenient?

So this is doable...however I think it will have a performance impact both because we need to call Object.getOwnPropertySymbols on the previous values, find the function in it (and we need to do that for every attachment) and we also need to call delete effects[that_symbol] (I know delete is veeeeery slow)...that said we should gain by not recreating an effect so maybe worth it?

I'll open a PR and we can discuss.

[Rich-Harris]

closing as wontfix per #16048 (comment)