fix test, add comment

Author: dummdidumm

packages/svelte/src/legacy/legacy-client.js

@@ -66,6 +66,9 @@ class Svelte4Component {
 	 * }} options
 	 */
 	constructor(options) {
+		// Using proxy state here isn't completely mirroring the Svelte 4 behavior, because mutations to a property
+		// cause fine-grained updates to only the places where that property is used, and not the entire property.
+		// Reactive statements and actions (the things where this matters) are handling this properly regardless, so it should be fine in practise.
 		const props = $.proxy({ ...(options.props || {}), $$events: this.#events }, false);
 		this.#instance = (options.hydrate ? $.hydrate : $.mount)(options.component, {
 			target: options.target,

packages/svelte/tests/runtime-legacy/samples/binding-backflow/_config.js

@@ -34,14 +34,14 @@ export default test({
 
 		p = parents['reactive_mutate'];
 		assert.deepEqual(p.value, { foo: 'kid' });
-		assert.equal(p.updates.length, 1);
+		assert.equal(p.updates.length, 2);
 
 		p = parents['init_update'];
 		assert.deepEqual(p.value, { foo: 'kid' });
 		assert.equal(p.updates.length, 2);
 
 		p = parents['init_mutate'];
 		assert.deepEqual(p.value, { foo: 'kid' });
-		assert.equal(p.updates.length, 1);
+		assert.equal(p.updates.length, 2);
 	}
 });