fix: ensure `$state.snapshot` never errors

Snapshotting can error on un-cloneable objects. It's not practical to error in this case; often there's no way out of this for users, so it makes sense to return the original value in that case, and warn in dev mode about it.

closes #12438

.changeset/brave-gorillas-fold.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure `$state.snapshot` never errors

packages/svelte/messages/shared-warnings/warnings.md

@@ -1,3 +1,7 @@
 ## dynamic_void_element_content
 
 > `<svelte:element this="%tag%">` is a void element — it cannot have content
+
+## state_snapshot_uncloneable
+
+> An object could not be cloned with $state.snapshot, the original value will be returned

packages/svelte/src/internal/shared/clone.js

@@ -1,4 +1,6 @@
 /** @import { Snapshot } from './types' */
+import { DEV } from 'esm-env';
+import * as w from './warnings.js';
 import { get_prototype_of, is_array, object_prototype } from './utils.js';
 
 /**
@@ -50,5 +52,18 @@ function clone(value, cloned) {
 		}
 	}
 
-	return /** @type {Snapshot<T>} */ (structuredClone(value));
+	if (value instanceof EventTarget) {
+		// can't be cloned
+		return /** @type {Snapshot<T>} */ (value);
+	}
+
+	try {
+		return /** @type {Snapshot<T>} */ (structuredClone(value));
+	} catch (e) {
+		if (DEV) {
+			w.state_snapshot_uncloneable();
+			console.warn(e);
+		}
+		return /** @type {Snapshot<T>} */ (value);
+	}
 }

packages/svelte/src/internal/shared/warnings.js

@@ -17,3 +17,15 @@ export function dynamic_void_element_content(tag) {
 		console.warn("dynamic_void_element_content");
 	}
 }
+
+/**
+ * An object could not be cloned with $state.snapshot, the original value will be returned
+ */
+export function state_snapshot_uncloneable() {
+	if (DEV) {
+		console.warn(`%c[svelte] state_snapshot_uncloneable\n%cAn object could not be cloned with $state.snapshot, the original value will be returned`, bold, normal);
+	} else {
+		// TODO print a link to the documentation
+		console.warn("state_snapshot_uncloneable");
+	}
+}