Unexpected behavior when combining spread operator with createAttachmentKey

[Jimdooz]

Describe the bug

When using the spread operator {...obj} to apply an object containing a property created with createAttachmentKey() to an element, the attachment is destroyed and recreated on every component re-render, even for unrelated state changes.

This behavior differs from directly using the @attach directive, where the attachment is properly maintained across re-renders.

Reproduction

<script>
    import { createAttachmentKey } from "svelte/attachments";

    function element() {
        let value = $state(false);

        const root = {
            [createAttachmentKey()]: () => {
                console.log("attached");

                return () => {
                    console.log("detached");
                }
            }
        };

        return {
            root,
            get value() { return value },
            set value(v) { value = v }
        }
    }

    const elem = element();
</script>

<div {...elem.root} {@attach () => {
    console.log("local attached");
    return () => {
        console.log("local detached");
    }
}}>
    <p>Value: {elem.value}</p>
</div>

<button onclick={() => elem.value = !elem.value}>Toggle</button>

Click multiple time on button, and check console

Expecting result

attached
local attached
*click*
*click*
*click*

Result obtained

attached
local attached
*click*
detached
attached
*click*
detached
attached
*click*
detached
attached

Logs

System Info

System:
    OS: Windows 11 10.0.26100
    CPU: (12) x64 Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
    Memory: 5.18 GB / 15.94 GB
  Binaries:
    Node: 22.12.0 - C:\Program Files\nodejs\node.EXE
    npm: 11.0.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.14.2 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (132.0.2957.127)
    Internet Explorer: 11.0.26100.1882

Severity

annoyance