Unrevert hydration optimisation & fix claim_text

Unreverts the hydration optimisation from sveltejs#6204 and fixes the `claim_text` issue as described in sveltejs#6279 (comment)

src/runtime/internal/Component.ts

@@ -1,7 +1,7 @@
 import { add_render_callback, flush, schedule_update, dirty_components } from './scheduler';
 import { current_component, set_current_component } from './lifecycle';
 import { blank_object, is_empty, is_function, run, run_all, noop } from './utils';
-import { children, detach } from './dom';
+import { children, detach, start_hydrating, end_hydrating } from './dom';
 import { transition_in } from './transitions';
 
 interface Fragment {
@@ -150,6 +150,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 
 	if (options.target) {
 		if (options.hydrate) {
+			start_hydrating();
 			const nodes = children(options.target);
 			// eslint-disable-next-line @typescript-eslint/no-non-null-assertion
 			$$.fragment && $$.fragment!.l(nodes);
@@ -161,6 +162,7 @@ export function init(component, options, instance, create_fragment, not_equal, p
 
 		if (options.intro) transition_in(component.$$.fragment);
 		mount_component(component, options.target, options.anchor, options.customElement);
+		end_hydrating();
 		flush();
 	}
 

src/runtime/internal/dom.ts

@@ -1,15 +1,47 @@
 import { has_prop } from './utils';
 
+// Track which nodes are claimed during hydration. Unclaimed nodes can then be removed from the DOM
+// at the end of hydration without touching the remaining nodes.
+let is_hydrating = false;
+const nodes_to_detach = new Set<Node>();
+
+export function start_hydrating() {
+	is_hydrating = true;
+}
+export function end_hydrating() {
+	is_hydrating = false;
+
+	for (const node of nodes_to_detach) {
+		node.parentNode.removeChild(node);
+	}
+
+	nodes_to_detach.clear();
+}
+
 export function append(target: Node, node: Node) {
-	target.appendChild(node);
+	if (is_hydrating) {
+		nodes_to_detach.delete(node);
+	}
+	if (node.parentNode !== target) {
+		target.appendChild(node);
+	}
 }
 
 export function insert(target: Node, node: Node, anchor?: Node) {
-	target.insertBefore(node, anchor || null);
+	if (is_hydrating) {
+		nodes_to_detach.delete(node);
+	}
+	if (node.parentNode !== target || (anchor && node.nextSibling !== anchor)) {
+		target.insertBefore(node, anchor || null);
+	}
 }
 
 export function detach(node: Node) {
-	node.parentNode.removeChild(node);
+	if (is_hydrating) {
+		nodes_to_detach.add(node);
+	} else if (node.parentNode) {
+		node.parentNode.removeChild(node);
+	}
 }
 
 export function destroy_each(iterations, detaching) {
@@ -154,8 +186,9 @@ export function children(element) {
 }
 
 export function claim_element(nodes, name, attributes, svg) {
-	for (let i = 0; i < nodes.length; i += 1) {
-		const node = nodes[i];
+	while (nodes.length > 0) {
+		const node = nodes.shift();
+
 		if (node.nodeName === name) {
 			let j = 0;
 			const remove = [];
@@ -168,7 +201,10 @@ export function claim_element(nodes, name, attributes, svg) {
 			for (let k = 0; k < remove.length; k++) {
 				node.removeAttribute(remove[k]);
 			}
-			return nodes.splice(i, 1)[0];
+
+			return node;
+		} else {
+			detach(node);
 		}
 	}
 
@@ -179,7 +215,12 @@ export function claim_text(nodes, data) {
 	for (let i = 0; i < nodes.length; i += 1) {
 		const node = nodes[i];
 		if (node.nodeType === 3) {
-			node.data = '' + data;
+			data = '' + data;
+			if (data.length < node.data.length) {
+				let newNode = node.splitText(data.length);
+				nodes[i] = newNode;
+				return node;
+			}
 			return nodes.splice(i, 1)[0];
 		}
 	}