fix: restore value after attribute removal during hydration

.changeset/loud-numbers-flow.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: restore value after attribute removal during hydration

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

@@ -16,8 +16,12 @@ import * as w from '../../warnings.js';
  */
 export function remove_input_attr_defaults(dom) {
 	if (hydrating) {
+		// using getAttribute instead of dome.value allow us to have
+		// null instead of "on" if the user didn't set a value
+		const value = dom.getAttribute('value');
 		set_attribute(dom, 'value', null);
 		set_attribute(dom, 'checked', null);
+		if (value) dom.value = value;
 	}
 }
 

packages/svelte/tests/runtime-browser/samples/fine-grained-hydration-clean-attr/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../assert';
+
+export default test({
+	html: `<input type="checkbox" name="lang" value="keep" />`,
+	ssrHtml: `<input type="checkbox" name="lang" value="keep" checked />`,
+	test({ window, assert, target, mod }) {
+		const input = target.querySelector('input');
+		assert.equal(input?.checked, true);
+	}
+});

packages/svelte/tests/runtime-browser/samples/fine-grained-hydration-clean-attr/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	let to_check = $state("keep")
+</script>
+
+<input type="checkbox" name="lang" value="keep" checked={to_check === "keep"} />