use hydration marker as effect boundary where possible

packages/svelte/src/internal/client/dom/blocks/html.js

@@ -2,7 +2,7 @@ import { derived } from '../../reactivity/deriveds.js';
 import { block, branch, destroy_effect, render_effect } from '../../reactivity/effects.js';
 import { get } from '../../runtime.js';
 import { is_array } from '../../utils.js';
-import { hydrate_nodes, hydrating } from '../hydration.js';
+import { hydrate_nodes, hydrate_start, hydrating } from '../hydration.js';
 import { create_fragment_from_html, remove } from '../reconciler.js';
 import { assign_nodes } from '../template.js';
 
@@ -31,7 +31,10 @@ export function html(anchor, get_value, svg, mathml) {
 
 		effect = branch(() => {
 			if (hydrating) {
-				assign_nodes(hydrate_nodes[0], hydrate_nodes[hydrate_nodes.length - 1]);
+				assign_nodes(
+					hydrate_start.previousSibling ?? hydrate_start,
+					hydrate_nodes[hydrate_nodes.length - 1]
+				);
 				return;
 			}
 

packages/svelte/src/internal/client/dom/blocks/if.js

@@ -3,7 +3,6 @@ import { hydrate_nodes, hydrating, set_hydrating } from '../hydration.js';
 import { remove } from '../reconciler.js';
 import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
 import { HYDRATION_END_ELSE } from '../../../../constants.js';
-import { assign_nodes } from '../template.js';
 
 /**
  * @param {Comment} anchor

packages/svelte/src/internal/client/dom/template.js

@@ -45,7 +45,7 @@ export function template(content, flags) {
 	return () => {
 		if (hydrating) {
 			assign_nodes(
-				has_start ? hydrate_start : unset ? undefined : null,
+				hydrate_start.previousSibling ?? hydrate_start,
 				hydrate_nodes[hydrate_nodes.length - 1]
 			);
 
@@ -117,7 +117,10 @@ export function ns_template(content, flags, ns = 'svg') {
 
 	return () => {
 		if (hydrating) {
-			assign_nodes(has_start ? hydrate_start : null, hydrate_nodes[hydrate_nodes.length - 1]);
+			assign_nodes(
+				hydrate_start.previousSibling ?? hydrate_start,
+				hydrate_nodes[hydrate_nodes.length - 1]
+			);
 
 			return hydrate_start;
 		}
@@ -248,7 +251,7 @@ export function comment(flags = 0) {
 	// we're not delegating to `template` here for performance reasons
 	if (hydrating) {
 		assign_nodes(
-			(flags & TEMPLATE_UNSET_START) !== 0 ? undefined : null,
+			hydrate_start.previousSibling ?? hydrate_start,
 			hydrate_nodes[hydrate_nodes.length - 1]
 		);