fix: ensure legacy props cache last value when destroyed

.changeset/khaki-carrots-mix.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure legacy props cache last value when destroyed

packages/svelte/src/internal/client/reactivity/props.js

@@ -324,12 +324,23 @@ export function prop(props, key, flags, fallback) {
 	} else {
 		// Svelte 4 did not trigger updates when a primitive value was updated to the same value.
 		// Replicate that behavior through using a derived
-		var derived_getter = with_parent_branch(() =>
-			(immutable ? derived : derived_safe_equal)(() => /** @type {V} */ (props[key]))
-		);
+		var derived_getter = with_parent_branch(() => {
+			return (immutable ? derived : derived_safe_equal)(() => /** @type {V} */ (props[key]));
+		});
+		// Connect the derived getter to the parent branch effect
+		get(derived_getter);
+
 		derived_getter.f |= LEGACY_DERIVED_PROP;
+		/** @type {V} */
+		var last_value;
+
 		getter = () => {
+			// Emulate the Svelte 4 behaviour of returning the last known value
+			if ((derived_getter.f & DESTROYED) !== 0) {
+				return last_value;
+			}
 			var value = get(derived_getter);
+			last_value = value;
 			if (value !== undefined) fallback_value = /** @type {V} */ (undefined);
 			return value === undefined ? fallback_value : value;
 		};

packages/svelte/tests/runtime-legacy/samples/props-reactive-destroy/Child.svelte

@@ -0,0 +1,9 @@
+<script>
+	import { onDestroy } from 'svelte';
+
+	export let data;
+	
+	onDestroy(() => {
+		data;
+	});
+</script>

packages/svelte/tests/runtime-legacy/samples/props-reactive-destroy/_config.js

@@ -0,0 +1,10 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	test({ assert, logs, target }) {
+		target.querySelector('button')?.click();
+		flushSync();
+		assert.deepEqual(logs, ['should fire once']);
+	}
+});

packages/svelte/tests/runtime-legacy/samples/props-reactive-destroy/main.svelte

@@ -0,0 +1,17 @@
+<script>
+	import Child from './Child.svelte';
+
+	let active = true;
+	let data = { example: 'This is some example data' };
+
+	function log(data) {
+		console.log('should fire once');
+		return data;
+	}
+</script>
+
+<button on:click={() => active = false}>Hide</button>
+
+{#if active}
+	<Child data={log(data)} />
+{/if}