fix, enhance test, explain

Author: dummdidumm

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

@@ -296,7 +296,8 @@ export function prop(props, key, flags, fallback) {
 	var is_entry_props = STATE_SYMBOL in props || LEGACY_PROPS in props;
 
 	var setter =
-		get_descriptor(props, key)?.set ?? (is_entry_props ? (v) => (props[key] = v) : undefined);
+		get_descriptor(props, key)?.set ??
+		(is_entry_props && bindable ? (v) => (props[key] = v) : undefined);
 
 	var fallback_value = /** @type {V} */ (fallback);
 	var fallback_dirty = true;
@@ -322,7 +323,7 @@ export function prop(props, key, flags, fallback) {
 		}
 
 		prop_value = get_fallback();
-		if (setter && (!is_entry_props || bindable)) setter(prop_value);
+		if (setter) setter(prop_value);
 	}
 
 	/** @type {() => V} */

packages/svelte/tests/runtime-runes/samples/mount-props-updates/_config.js

@@ -5,8 +5,9 @@ export default test({
 	test({ assert, target }) {
 		assert.htmlEqual(
 			target.innerHTML,
+			// Even thought we're in runes mode, the buz fallback propagates back up in this case
 			`
-			<button>reset</button>
+			<button>reset</button> foo baz buz
 			<div><button>update</button> foo bar baz buz</div>
 			<div><button>update</button> foo bar baz buz</div>
 			`
@@ -19,8 +20,9 @@ export default test({
 		flushSync();
 		assert.htmlEqual(
 			target.innerHTML,
+			// bar is not set in the parent because it's a readonly property
 			`
-			<button>reset</button>
+			<button>reset</button> foo 3 4
 			<div><button>update</button> 1 2 3 4</div>
 			<div><button>update</button> 1 2 3 4</div>
 			`
@@ -30,10 +32,13 @@ export default test({
 		flushSync();
 		assert.htmlEqual(
 			target.innerHTML,
+			// Because foo is a readonly property, component.svelte diverges locally from it,
+			// and the passed in property keeps the initial value of foo. This is why it stays
+			// at 1, because foo is not updated to a different value.
 			`
-				<button>reset</button>
-			<div><button>update</button> foo bar baz buz</div>
-			<div><button>update</button> foo bar baz buz</div>
+				<button>reset</button> foo bar baz buz
+			<div><button>update</button> 1 bar baz buz</div>
+			<div><button>update</button> 1 bar baz buz</div>
 				`
 		);
 	}

packages/svelte/tests/runtime-runes/samples/mount-props-updates/main.svelte

@@ -25,6 +25,6 @@
 		props.baz = 'baz';
 		props.buz = 'buz';
 	}}>reset</button
->
+> {props.foo} {props.bar} {props.baz} {props.buz}
 <div bind:this={div1}></div>
 <div bind:this={div2}></div>