fix: batch resolution of async work (#17511)

* fix: batch resolution of async work

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

* fix

* drive-by tidy up — Batch.enqueue is (apparently?) a useless wrapper around queue_micro_task

* 99% sure it's impossible for is_flushing to be true inside a queued task

* add some comments

* return early, reduce indentation

---------

Co-authored-by: David Roizenman <hmnd@users.noreply.github.com>

.changeset/four-signs-listen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: batch resolution of async work

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

@@ -102,6 +102,7 @@ export class Boundary {
 
 	#local_pending_count = 0;
 	#pending_count = 0;
+	#pending_count_update_queued = false;
 
 	#is_creating_fallback = false;
 
@@ -202,12 +203,11 @@ export class Boundary {
 
 	#hydrate_pending_content() {
 		const pending = this.#props.pending;
-		if (!pending) {
-			return;
-		}
+		if (!pending) return;
+
 		this.#pending_effect = branch(() => pending(this.#anchor));
 
-		Batch.enqueue(() => {
+		queue_micro_task(() => {
 			var anchor = this.#get_anchor();
 
 			this.#main_effect = this.#run(() => {
@@ -359,9 +359,15 @@ export class Boundary {
 
 		this.#local_pending_count += d;
 
-		if (this.#effect_pending) {
-			internal_set(this.#effect_pending, this.#local_pending_count);
-		}
+		if (!this.#effect_pending || this.#pending_count_update_queued) return;
+		this.#pending_count_update_queued = true;
+
+		queue_micro_task(() => {
+			this.#pending_count_update_queued = false;
+			if (this.#effect_pending) {
+				internal_set(this.#effect_pending, this.#local_pending_count);
+			}
+		});
 	}
 
 	get_effect_pending() {

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

@@ -138,6 +138,8 @@ export class Batch {
 
 	is_fork = false;
 
+	#decrement_queued = false;
+
 	is_deferred() {
 		return this.is_fork || this.#blocking_pending > 0;
 	}
@@ -426,7 +428,22 @@ export class Batch {
 		this.#pending -= 1;
 		if (blocking) this.#blocking_pending -= 1;
 
-		this.revive();
+		if (this.#decrement_queued) return;
+		this.#decrement_queued = true;
+
+		queue_micro_task(() => {
+			this.#decrement_queued = false;
+
+			if (!this.is_deferred()) {
+				// we only reschedule previously-deferred effects if we expect
+				// to be able to run them after processing the batch
+				this.revive();
+			} else if (queued_root_effects.length > 0) {
+				// if other effects are scheduled, process the batch _without_
+				// rescheduling the previously-deferred effects
+				this.flush();
+			}
+		});
 	}
 
 	revive() {
@@ -464,7 +481,7 @@ export class Batch {
 			batches.add(current_batch);
 
 			if (!is_flushing_sync) {
-				Batch.enqueue(() => {
+				queue_micro_task(() => {
 					if (current_batch !== batch) {
 						// a flushSync happened in the meantime
 						return;
@@ -478,11 +495,6 @@ export class Batch {
 		return current_batch;
 	}
 
-	/** @param {() => void} task */
-	static enqueue(task) {
-		queue_micro_task(task);
-	}
-
 	apply() {
 		if (!async_mode_flag || (!this.is_fork && batches.size === 1)) return;