fix: improve bind:this support around proxyied state

.changeset/blue-ants-raise.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: improve bind:this support around proxyied state

packages/svelte/src/internal/client/render.js

@@ -77,6 +77,7 @@ import { run } from '../common.js';
 import { bind_transition, trigger_transitions } from './transitions.js';
 import { mutable_source, source, set } from './reactivity/sources.js';
 import { safe_equal, safe_not_equal } from './reactivity/equality.js';
+import { STATE_SYMBOL } from './constants.js';
 
 /** @type {Set<string>} */
 const all_registerd_events = new Set();
@@ -1336,6 +1337,17 @@ export function bind_prop(props, prop, value) {
 	}
 }
 
+/**
+ * @param {any} bound_value
+ * @param {Element} element_or_component
+ * @returns {boolean}
+ */
+function is_bound_this(bound_value, element_or_component) {
+	// Find the original target if the value is proxied.
+	const proxy_target = bound_value && bound_value[STATE_SYMBOL]?.t;
+	return bound_value === element_or_component || proxy_target === element_or_component;
+}
+
 /**
  * @param {Element} element_or_component
  * @param {(value: unknown, ...parts: unknown[]) => void} update
@@ -1360,7 +1372,7 @@ export function bind_this(element_or_component, update, get_value, get_parts) {
 				update(element_or_component, ...parts);
 				// If this is an effect rerun (cause: each block context changes), then nullfiy the binding at
 				// the previous position if it isn't already taken over by a different effect.
-				if (old_parts && get_value(...old_parts) === element_or_component) {
+				if (old_parts && is_bound_this(get_value(...old_parts), element_or_component)) {
 					update(null, ...old_parts);
 				}
 			}
@@ -1374,7 +1386,7 @@ export function bind_this(element_or_component, update, get_value, get_parts) {
 		// Defer to the next tick so that all updates can be reconciled first.
 		// This solves the case where one variable is shared across multiple this-bindings.
 		effect(() => {
-			if (get_value(...parts) === element_or_component) {
+			if (parts && is_bound_this(get_value(...parts), element_or_component)) {
 				update(null, ...parts);
 			}
 		});

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

@@ -0,0 +1,4 @@
+<script>
+	export const a = {};
+</script>
+<div>Hello world</div>

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

@@ -0,0 +1,29 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+import { log } from './log.js';
+
+export default test({
+	before_test: () => {
+		log.length = 0;
+	},
+
+	html: `<button>Toggle</button><div>Hello\nworld</div>`,
+
+	async test({ assert, target }) {
+		const [btn1] = target.querySelectorAll('button');
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>Toggle</button>`);
+
+		flushSync(() => {
+			btn1?.click();
+		});
+
+		assert.htmlEqual(target.innerHTML, `<button>Toggle</button><div>Hello\nworld</div>`);
+
+		assert.deepEqual(log, [{ a: {} }, null, { a: {} }]);
+	}
+});

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

@@ -0,0 +1,2 @@
+/** @type {any[]} */
+export const log = [];

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

@@ -0,0 +1,23 @@
+<script>
+	import { log } from './log.js';
+	import Component from './Component.svelte';
+
+	let type = $state(Component)
+	let elem = $state()
+
+	$effect(() => {
+		log.push(elem);
+	});
+</script>
+
+<button onclick={() => {
+	if (!type) {
+		type = Component
+	} else {
+		type = false
+	}
+}}>Toggle</button>
+
+<svelte:component bind:this={elem} this={type}>
+	Content
+</svelte:component>