fix: account for proxified instance when updating `bind:this` (#18147)

Fixes #18145

I wonder why nulling happens after updating `bind:this` but not before,
which would fix the issue as well, though not as efficiently.

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- [x] 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:
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- [x] Prefix your PR title with `feat:`, `fix:`, `chore:`, or `docs:`.
- [x] This message body should clearly illustrate what problems it
solves.
- [x] Ideally, include a test that fails without this PR but passes with
it.
- [x] If this PR changes code within `packages/svelte/src`, add a
changeset (`npx changeset`).

### Tests and linting

- [x] Run the tests with `pnpm test` and lint the project with `pnpm
lint`

.changeset/full-waves-tease.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: account for proxified instance when updating `bind:this`

packages/svelte/src/internal/client/dom/elements/bindings/this.js

@@ -40,7 +40,7 @@ export function bind_this(element_or_component = {}, update, get_value, get_part
 			parts = get_parts?.() || [];
 
 			untrack(() => {
-				if (element_or_component !== get_value(...parts)) {
+				if (!is_bound_this(get_value(...parts), element_or_component)) {
 					update(element_or_component, ...parts);
 					// If this is an effect rerun (cause: each block context changes), then nullify the binding at
 					// the previous position if it isn't already taken over by a different effect.

packages/svelte/tests/runtime-runes/samples/bind-this-proxy-deep/Component.svelte

@@ -0,0 +1,5 @@
+<script>
+	const props = $props();
+	// svelte-ignore state_referenced_locally
+	export const name = props.name;
+</script>

packages/svelte/tests/runtime-runes/samples/bind-this-proxy-deep/_config.js

@@ -0,0 +1,22 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ assert, target, logs }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		flushSync(() => {
+			btn?.click();
+		});
+
+		assert.deepEqual(logs, [
+			{},
+			{ 0: { name: 'Row 0' } },
+			{ 0: { name: 'Row 0' }, 1: { name: 'Row 1' } }
+		]);
+	}
+});

packages/svelte/tests/runtime-runes/samples/bind-this-proxy-deep/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import Row from "./Component.svelte";
+
+	const nums = $state([]);
+	const rows = $derived(nums.map(n => ({id: n, name: `Row ${n}` })));
+	const refs = $state({});
+
+	$effect(() => {
+		console.log({...refs});
+	})
+</script>
+
+<button onclick={() => nums.push(nums.length)}>Add</button>
+{#each rows as row (row.id)}
+	<Row name={row.name} bind:this={refs[row.id]} />
+{/each}