$derived does not update on attribute addition

[jerrythomas]

Describe the bug

Something seems to have changed recently in the way $derived works.
This test below used to update the lookup on addition of children attribute.

it.only('should handle a nested array', () => {
		const items = $state([{ text: 'A' }, { text: 'B' }, { text: 'C' }])
		const lookup = $derived(deriveLookupWithProxy(items))
		const result = () => lookup
		expect(Array.from(result().keys())).toEqual(['0', '1', '2'])

		const addChildren = () => {
			items[0].children = [{ text: 'A1' }, { text: 'A2' }]
		}
		addChildren()
		// flushSync()
		expect(Array.from(result().keys())).toEqual(['0', '0-0', '0-1', '1', '2'])
	})

I still see that the function is called on the change, but it does not seem to update the values. This used to work recently.

Reproduction

import { describe, it, expect } from 'vitest'
import { isNil, has } from 'ramda'
// import { flushSync } from 'svelte'

const defaultFields = { children: 'children' }

class Proxy {
	constructor(value, fields) {
		this.fields = fields
		this.value = value
	}
	get(fieldName, defaultValue) {
		return this.has(fieldName) ? this.value[this.fields[fieldName]] : defaultValue
	}
	has(fieldName) {
		const mappedField = this.fields[fieldName]
		return !isNil(mappedField) && has(mappedField, this.value)
	}

	get hasChildren() {
		return this.has('children') && Array.isArray(this.value[this.fields.children])
	}
}

export function deriveLookupWithProxy(items, fields = defaultFields, path = []) {
	const lookup = new Map()
	console.log($state.snapshot(items))

	items.forEach((item, index) => {
		const itemPath = [...path, index]
		const key = itemPath.join('-')
		const proxy = new Proxy(item, fields)

		lookup.set(key, proxy)

		if (proxy.hasChildren) {
			const childLookup = deriveLookupWithProxy(proxy.get('children'), fields, itemPath)
			for (const [childKey, childValue] of childLookup.entries()) {
				lookup.set(childKey, childValue)
			}
		}
	})
	return lookup
}

describe('deriveLookupWithProxy', () => {
	it.only('should handle a nested array', () => {
		const items = $state([{ text: 'A' }, { text: 'B' }, { text: 'C' }])
		const lookup = $derived(deriveLookupWithProxy(items))
		const result = () => lookup
		expect(Array.from(result().keys())).toEqual(['0', '1', '2'])

		const addChildren = () => {
			items[0].children = [{ text: 'A1' }, { text: 'A2' }]
		}
		addChildren()
		// flushSync()
		expect(Array.from(result().keys())).toEqual(['0', '0-0', '0-1', '1', '2'])
	})
})

Logs

System Info

System:
    OS: macOS 15.4.1
    CPU: (16) arm64 Apple M4 Max
    Memory: 640.27 MB / 48.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 23.4.0 - ~/.nvm/versions/node/v23.4.0/bin/node
    npm: 10.9.2 - ~/.nvm/versions/node/v23.4.0/bin/npm
    pnpm: 10.7.0 - ~/Library/pnpm/pnpm
    bun: 1.2.12 - /opt/homebrew/bin/bun
  Browsers:
    Chrome: 136.0.7103.93
    Safari: 18.4

Severity

blocking an upgrade