diff --git a/packages/core/test/lib/utils/promisebuffer.test.ts b/packages/core/test/lib/utils/promisebuffer.test.ts
index 9c944ffd0c39..d1b4b9abc48d 100644
--- a/packages/core/test/lib/utils/promisebuffer.test.ts
+++ b/packages/core/test/lib/utils/promisebuffer.test.ts
@@ -1,8 +1,11 @@
-import { describe, expect, test, vi } from 'vitest';
+import { afterEach, describe, expect, test, vi } from 'vitest';
 import { makePromiseBuffer } from '../../../src/utils/promisebuffer';
 import { rejectedSyncPromise, resolvedSyncPromise } from '../../../src/utils/syncpromise';
 
 describe('PromiseBuffer', () => {
+  afterEach(() => {
+    vi.useRealTimers();
+  });
   describe('add()', () => {
     test('enforces limit of promises', async () => {
       const buffer = makePromiseBuffer(5);
@@ -105,20 +108,28 @@ describe('PromiseBuffer', () => {
 
   describe('drain()', () => {
     test('drains all promises without timeout', async () => {
+      vi.useFakeTimers();
+
       const buffer = makePromiseBuffer();
 
-      const p1 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 1)));
-      const p2 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 1)));
-      const p3 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 1)));
-      const p4 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 1)));
-      const p5 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 1)));
+      const p1 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
+      const p2 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
+      const p3 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
+      const p4 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
+      const p5 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
 
       [p1, p2, p3, p4, p5].forEach(p => {
         void buffer.add(p);
       });
 
       expect(buffer.$.length).toEqual(5);
-      const result = await buffer.drain();
+
+      const drainPromise = buffer.drain();
+
+      // Advance time to resolve all promises
+      await vi.advanceTimersByTimeAsync(10);
+
+      const result = await drainPromise;
       expect(result).toEqual(true);
       expect(buffer.$.length).toEqual(0);
 
@@ -130,13 +141,15 @@ describe('PromiseBuffer', () => {
     });
 
     test('drains all promises with timeout', async () => {
+      vi.useFakeTimers();
+
       const buffer = makePromiseBuffer();
 
-      const p1 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 2)));
-      const p2 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 4)));
-      const p3 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 6)));
-      const p4 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 8)));
-      const p5 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 10)));
+      const p1 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 20)));
+      const p2 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 40)));
+      const p3 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 60)));
+      const p4 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 80)));
+      const p5 = vi.fn(() => new Promise(resolve => setTimeout(resolve, 100)));
 
       [p1, p2, p3, p4, p5].forEach(p => {
         void buffer.add(p);
@@ -149,15 +162,29 @@ describe('PromiseBuffer', () => {
       expect(p5).toHaveBeenCalled();
 
       expect(buffer.$.length).toEqual(5);
-      const result = await buffer.drain(6);
+
+      // Start draining with a 50ms timeout
+      const drainPromise = buffer.drain(50);
+
+      // Advance time by 50ms - this will:
+      // - Resolve p1 (20ms) and p2 (40ms)
+      // - Trigger the drain timeout (50ms)
+      // - p3, p4, p5 are still pending
+      await vi.advanceTimersByTimeAsync(50);
+
+      const result = await drainPromise;
       expect(result).toEqual(false);
-      // p5 & p4 are still in the buffer
-      // Leaving some wiggle room, possibly one or two items are still in the buffer
-      // to avoid flakiness
-      expect(buffer.$.length).toBeGreaterThanOrEqual(1);
 
-      // Now drain final item
-      const result2 = await buffer.drain();
+      // p3, p4 & p5 are still in the buffer
+      expect(buffer.$.length).toEqual(3);
+
+      // Now drain remaining items without timeout
+      const drainPromise2 = buffer.drain();
+
+      // Advance time to resolve remaining promises
+      await vi.advanceTimersByTimeAsync(100);
+
+      const result2 = await drainPromise2;
       expect(result2).toEqual(true);
       expect(buffer.$.length).toEqual(0);
     });