fix: avoid microtask in flushSync

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

@@ -59,6 +59,7 @@ let queued_root_effects = [];
 let last_scheduled_effect = null;
 
 let is_flushing = false;
+let in_flush_sync = false;
 
 export class Batch {
 	/**
@@ -421,6 +422,7 @@ export class Batch {
 			const batch = (current_batch = new Batch());
 			batches.add(current_batch);
 
+			if (!in_flush_sync) {
 				queueMicrotask(() => {
 					if (current_batch !== batch) {
 						// a flushSync happened in the meantime
@@ -430,6 +432,7 @@ export class Batch {
 					batch.flush();
 				});
 			}
+		}
 
 		return current_batch;
 	}
@@ -443,6 +446,8 @@ export class Batch {
  * @returns {T}
  */
 export function flushSync(fn) {
+	in_flush_sync = true;
+
 	if (async_mode_flag && active_effect !== null) {
 		e.flush_sync_in_effect();
 	}
@@ -473,6 +478,8 @@ export function flushSync(fn) {
 				dev_effect_stack = [];
 			}
 
+			in_flush_sync = false;
+
 			return /** @type {T} */ (result);
 		}