fix: reflect all custom element prop updates back to attribute

fixes #8879

.changeset/gold-tips-roll.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reflect all custom element prop updates back to attribute

packages/svelte/src/runtime/internal/Component.js

@@ -269,6 +269,31 @@ if (typeof HTMLElement === 'function') {
 						}
 					}
 				});
+
+				// Reflect component props as attributes
+				const reflect_attributes = () => {
+					this.$$r = true;
+					for (const key in this.$$p_d) {
+						this.$$d[key] = this.$$c.$$.ctx[this.$$c.$$.props[key]];
+						if (this.$$p_d[key].reflect) {
+							const attribute_value = get_custom_element_value(
+								key,
+								this.$$d[key],
+								this.$$p_d,
+								'toAttribute'
+							);
+							if (attribute_value == null) {
+								this.removeAttribute(key);
+							} else {
+								this.setAttribute(this.$$p_d[key].attribute || key, attribute_value);
+							}
+						}
+					}
+					this.$$r = false;
+				};
+				this.$$c.$$.after_update.push(reflect_attributes);
+				reflect_attributes(); // once initially because after_update is added too late for first render
+
 				for (const type in this.$$l) {
 					for (const listener of this.$$l[type]) {
 						const unsub = this.$$c.$on(type, listener);
@@ -386,21 +411,6 @@ export function create_custom_element(
 				value = get_custom_element_value(prop, value, props_definition);
 				this.$$d[prop] = value;
 				this.$$c?.$set({ [prop]: value });
-				if (props_definition[prop].reflect) {
-					this.$$r = true;
-					const attribute_value = get_custom_element_value(
-						prop,
-						value,
-						props_definition,
-						'toAttribute'
-					);
-					if (attribute_value == null) {
-						this.removeAttribute(prop);
-					} else {
-						this.setAttribute(props_definition[prop].attribute || prop, attribute_value);
-					}
-					this.$$r = false;
-				}
 			}
 		});
 	});

packages/svelte/test/runtime-browser/custom-elements-samples/reflect-attributes/main.svelte

@@ -1,19 +1,20 @@
 <svelte:options
 	customElement={{
-		tag: "custom-element",
-		props: { red: { reflect: true, type: "Boolean" } },
+		tag: 'custom-element',
+		props: { red: { reflect: true, type: 'Boolean' } }
 	}}
 />
 
 <script>
-	import "./my-widget.svelte";
+	import './my-widget.svelte';
 	export let red;
 	red;
 </script>
 
 <div>hi</div>
 <p>hi</p>
-<my-widget red white />
+<button on:click={() => (red = false)}>off</button>
+<my-widget {red} white />
 
 <style>
 	:host([red]) div {

packages/svelte/test/runtime-browser/custom-elements-samples/reflect-attributes/test.js

@@ -9,6 +9,7 @@ export default async function (target) {
 	const ceRoot = target.querySelector('custom-element').shadowRoot;
 	const div = ceRoot.querySelector('div');
 	const p = ceRoot.querySelector('p');
+	const button = ceRoot.querySelector('button');
 
 	assert.equal(getComputedStyle(div).color, 'rgb(255, 0, 0)');
 	assert.equal(getComputedStyle(p).color, 'rgb(255, 255, 255)');
@@ -19,4 +20,11 @@ export default async function (target) {
 
 	assert.equal(getComputedStyle(innerDiv).color, 'rgb(255, 0, 0)');
 	assert.equal(getComputedStyle(innerP).color, 'rgb(255, 255, 255)');
+
+	button.click();
+	await tick();
+	await tick();
+
+	assert.equal(getComputedStyle(div).color, 'rgb(0, 0, 0)');
+	assert.equal(getComputedStyle(innerDiv).color, 'rgb(0, 0, 0)');
 }