fix: prevent batches from getting intertwined (#16446)

* ensure batches are kept separate

* changeset

* activate batch before decrementing

Author: Rich-Harris

.changeset/fresh-birds-jam.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: prevent batches from getting intertwined

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

@@ -148,7 +148,7 @@ export class Boundary {
 				// need to use hydration boundary comments to report whether
 				// the pending or main block was rendered for a given
 				// boundary, and hydrate accordingly
-				queueMicrotask(() => {
+				Batch.enqueue(() => {
 					this.#main_effect = this.#run(() => {
 						Batch.ensure();
 						return branch(() => this.#children(this.#anchor));

packages/svelte/src/internal/client/reactivity/batch.js

@@ -49,6 +49,19 @@ export let batch_deriveds = null;
 /** @type {Set<() => void>} */
 export let effect_pending_updates = new Set();
 
+/** @type {Array<() => void>} */
+let tasks = [];
+
+function dequeue() {
+	const task = /** @type {() => void} */ (tasks.shift());
+
+	if (tasks.length > 0) {
+		queueMicrotask(dequeue);
+	}
+
+	task();
+}
+
 /** @type {Effect[]} */
 let queued_root_effects = [];
 
@@ -438,7 +451,7 @@ export class Batch {
 			batches.add(current_batch);
 
 			if (autoflush) {
-				queueMicrotask(() => {
+				Batch.enqueue(() => {
 					if (current_batch !== batch) {
 						// a flushSync happened in the meantime
 						return;
@@ -451,6 +464,15 @@ export class Batch {
 
 		return current_batch;
 	}
+
+	/** @param {() => void} task */
+	static enqueue(task) {
+		if (tasks.length === 0) {
+			queueMicrotask(dequeue);
+		}
+
+		tasks.unshift(task);
+	}
 }
 
 /**
@@ -593,7 +615,11 @@ export function suspend() {
 
 	return function unsuspend() {
 		boundary.update_pending_count(-1);
-		if (!pending) batch.decrement();
+
+		if (!pending) {
+			batch.activate();
+			batch.decrement();
+		}
 
 		unset_context();
 	};