refine approach, add test

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -146,7 +146,7 @@ export function set_xlink_attribute(dom, attribute, value) {
 }
 
 /**
- * @param {any} node
+ * @param {HTMLElement} node
  * @param {string} prop
  * @param {any} value
  */
@@ -161,10 +161,21 @@ export function set_custom_element_data(node, prop, value) {
 	set_active_reaction(null);
 	set_active_effect(null);
 	try {
-		if (get_setters(node).includes(prop)) {
+		if (
+			// Don't compute setters for custom elements while they aren't registered yet,
+			// because during their upgrade/instantiation they might add more setters.
+			// Instead, fall back to a simple "an object, then set as property" heuristic.
+			setters_cache.has(node.nodeName) || customElements.get(node.tagName.toLowerCase())
+				? get_setters(node).includes(prop)
+				: value && typeof value === 'object'
+		) {
+			// @ts-expect-error
 			node[prop] = value;
 		} else {
-			set_attribute(node, prop, value);
+			// We did getters etc checks already, stringify before passing to set_attribute
+			// to ensure it doesn't invoke the same logic again, and potentially populating
+			// the setters cache too early.
+			set_attribute(node, prop, value == null ? value : String(value));
 		}
 	} finally {
 		set_active_reaction(previous_reaction);
@@ -323,21 +334,11 @@ var setters_cache = new Map();
 
 /** @param {Element} element */
 function get_setters(element) {
-	var name = element.nodeName;
-	var setters = setters_cache.get(name);
-
-	if (setters) return setters;
-
-	setters = [];
-
-	// Don't cache the result for custom elements while they aren't connected yet,
-	// because during their upgrade they might add more setters
-	if (!name.includes('-') || element.isConnected) {
-		setters_cache.set(name, setters);
-	}
+	var setters = setters_cache.get(element.nodeName);
+	setters_cache.set(element.nodeName, (setters = []));
 
 	var descriptors;
-	var proto = element;
+	var proto = element; // In the case of custom elements there might be setters on the instance
 	var element_proto = Element.prototype;
 
 	// Stop at Element, from there on there's only unnecessary setters we're not interested in

packages/svelte/tests/runtime-browser/custom-elements-samples/late-ce-mount/_config.js

@@ -0,0 +1,25 @@
+import { flushSync } from 'svelte';
+import { test } from '../../assert';
+
+const tick = () => Promise.resolve();
+
+// Check that rendering a custom element and setting a property before it is registered
+// does not break the "when to set this as a property" logic
+export default test({
+	async test({ assert, target }) {
+		target.innerHTML = '<custom-element></custom-element>';
+		await tick();
+		await tick();
+
+		const ce_root = target.querySelector('custom-element').shadowRoot;
+
+		ce_root.querySelector('button')?.click();
+		flushSync();
+		await tick();
+		await tick();
+
+		const inner_ce_root = ce_root.querySelectorAll('set-property-before-mounted');
+		assert.htmlEqual(inner_ce_root[0].shadowRoot.innerHTML, 'object|{"foo":"bar"}');
+		assert.htmlEqual(inner_ce_root[1].shadowRoot.innerHTML, 'object|{"foo":"bar"}');
+	}
+});

packages/svelte/tests/runtime-browser/custom-elements-samples/late-ce-mount/main.svelte

@@ -0,0 +1,31 @@
+<svelte:options customElement="custom-element" />
+
+<script lang="ts">
+	import { onMount } from 'svelte';
+
+	class CustomElement extends HTMLElement {
+		constructor() {
+			super();
+			this.attachShadow({ mode: 'open' });
+			Object.defineProperty(this, 'property', {
+				set: (value) => {
+					this.shadowRoot.innerHTML = typeof value + '|' + JSON.stringify(value);
+				}
+			});
+		}
+	}
+
+	onMount(async () => {
+		customElements.define('set-property-before-mounted', CustomElement);
+	});
+
+	let property = $state();
+</script>
+
+<button onclick={() => (property = { foo: 'bar' })}>Update</button>
+<!-- one that's there before it's registered -->
+<set-property-before-mounted {property}></set-property-before-mounted>
+<!-- and one that's after registration but sets property to an object right away -->
+{#if property}
+	<set-property-before-mounted {property}></set-property-before-mounted>
+{/if}