fix: hydration comments during hmr (#17975)

Closes #17972

Claude found this fix I had a look and I think it makes sense (we are
injecting a new comment which messes up the marching during hydration).

I'm slightly confused why this only applied to `css_props` but I guess
that's because there's an "hidden" element there which makes it special.

I don't super-like the `if` situation, but I guess it is what it is.

Also the test was using `hmr` without `dev` and this brought to my
attention that we were just assuming components return something while
it's not the case...I guess it doesn't really matter unless you are
using `hmr` in prod which is not a thing but fixing this is simple so we
might just as well doing it

.changeset/calm-mice-allow.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: hydration comments during hmr

packages/svelte/src/internal/client/dev/hmr.js

@@ -34,7 +34,13 @@ export function hmr(fn) {
 		let start = create_comment();
 		let end = create_comment();
 
-		anchor.before(start);
+		// During hydration, inserting the start comment before the anchor could
+		// corrupt the DOM tree that the hydration walker is navigating (e.g. when
+		// a component is inside a CSS props wrapper gh-issue#17972). We defer the insertion until
+		// after the component has hydrated.
+		if (!hydrating) {
+			anchor.before(start);
+		}
 
 		block(() => {
 			if (component === (component = get(current))) {
@@ -52,13 +58,13 @@ export function hmr(fn) {
 				if (ran) set_should_intro(false);
 
 				// preserve getters/setters
-				Object.defineProperties(
-					instance,
-					Object.getOwnPropertyDescriptors(
-						// @ts-expect-error
-						new.target ? new component(anchor, props) : component(anchor, props)
-					)
-				);
+				var result =
+					// @ts-expect-error
+					new.target ? new component(anchor, props) : component(anchor, props);
+				// a component is not guaranteed to return something and we can't invoke getOwnPropertyDescriptors on undefined
+				if (result) {
+					Object.defineProperties(instance, Object.getOwnPropertyDescriptors(result));
+				}
 
 				if (ran) set_should_intro(true);
 			});
@@ -67,6 +73,9 @@ export function hmr(fn) {
 		ran = true;
 
 		if (hydrating) {
+			// Insert start comment now that hydration is done, so it doesn't
+			// corrupt the hydration walk
+			anchor.before(start);
 			anchor = hydrate_node;
 		}
 

packages/svelte/tests/hydration/samples/css-props-hmr/Component.svelte

@@ -0,0 +1,7 @@
+<h1>Hello</h1>
+
+<style>
+	h1 {
+		color: var(--color);
+	}
+</style>

packages/svelte/tests/hydration/samples/css-props-hmr/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		hmr: true
+	}
+});

packages/svelte/tests/hydration/samples/css-props-hmr/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	import Component from "./Component.svelte";
+</script>
+
+<Component --color="red" />