diff --git a/tests/scenarios/watch-mode-test.ts b/tests/scenarios/watch-mode-test.ts
index 8d87d5fbe..f0ec5a6bb 100644
--- a/tests/scenarios/watch-mode-test.ts
+++ b/tests/scenarios/watch-mode-test.ts
@@ -16,200 +16,101 @@ let app = appScenarios.map('watch-mode', () => {
    */
 });
 
-abstract class Waiter {
-  readonly promise: Promise<void>;
-  protected _resolve!: () => void;
-  protected _reject!: (error: unknown) => void;
-  private _timeout = (timeout: number) => this.onTimeout(timeout);
-
-  constructor(timeout: number | null = DEFAULT_TIMEOUT) {
-    this.promise = new Promise<void>((resolve, reject) => {
-      this._resolve = resolve;
-      this._reject = reject;
-    });
-
-    if (timeout !== null) {
-      setTimeout(() => this._timeout(timeout), timeout);
-    }
-  }
-
-  abstract onOutputLine(data: string): boolean;
-  abstract onExit(code: number): void;
-  abstract onTimeout(timeout: number): void;
-
-  protected resolve(): void {
-    const resolve = this._resolve;
-    this._resolve = this._reject = this._timeout = () => {};
-    resolve();
-  }
-
-  protected reject(error: unknown): void {
-    const reject = this._reject;
-    this._resolve = this._reject = this._timeout = () => {};
-    reject(error);
-  }
-}
-
 const DEFAULT_TIMEOUT = process.env.CI ? 90000 : 30000;
 
-class OutputWaiter extends Waiter {
-  constructor(private process: EmberCLI, private output: string | RegExp, timeout?: number | null) {
-    super(timeout);
-  }
-
-  onOutputLine(line: string): boolean {
-    if (this.matchLine(line)) {
-      this.resolve();
-      return true;
-    } else {
-      return false;
-    }
-  }
-
-  onExit(code: number): void {
-    try {
-      throw new Error(
-        'Process exited with code ' +
-          code +
-          ' before output "' +
-          this.output +
-          '" was found. ' +
-          'Recent output:\n\n' +
-          this.process.recentOutput
-      );
-    } catch (error) {
-      this.reject(error);
-    }
-  }
-
-  onTimeout(timeout: number): void {
-    try {
-      throw new Error(
-        'Timed out after ' +
-          timeout +
-          'ms before output "' +
-          this.output +
-          '" was found. ' +
-          'Recent output:\n\n' +
-          this.process.recentOutput
-      );
-    } catch (error) {
-      this.reject(error);
-    }
-  }
-
-  private matchLine(line: string): boolean {
-    if (typeof this.output === 'string') {
-      return this.output === line;
-    } else {
-      return this.output.test(line);
-    }
-  }
-}
-
-type Status = { type: 'running' } | { type: 'errored'; error: unknown } | { type: 'completed' };
-
 class EmberCLI {
   static launch(args: readonly string[], options: Options<string> = {}): EmberCLI {
     return new EmberCLI(execa('ember', args, { ...options, all: true }));
   }
 
-  readonly completed: Promise<void>;
-
-  private status: Status = { type: 'running' };
-  private waiters: Waiter[] = [];
   private lines: string[] = [];
+  private nextWaitedLine = 0;
+  private exitCode: number | null = null;
+  private currentWaiter: (() => void) | undefined;
 
   constructor(private process: ExecaChildProcess) {
     process.all!.on('data', data => {
       const lines = data.toString().split(/\r?\n/);
       this.lines.push(...lines);
-      for (const line of lines) {
-        this.waiters = this.waiters.filter(waiter => !waiter.onOutputLine(line));
-      }
+      this.currentWaiter?.();
     });
 
     process.on('exit', code => {
-      for (const waiter of this.waiters) {
-        waiter.onExit(code ?? 0);
-      }
-
-      this.waiters = [];
+      this.exitCode = code;
+      this.currentWaiter?.();
     });
-
-    const exit = new (class ExitWaiter extends Waiter {
-      constructor(private process: EmberCLI) {
-        super(null);
-      }
-
-      onOutputLine(): boolean {
-        return false;
-      }
-
-      onExit(code: number): void {
-        if (code === 0) {
-          this.resolve();
-        } else {
-          try {
-            throw new Error(
-              'Process exited with code ' + code + '. ' + 'Recent output:\n\n' + this.process.recentOutput
-            );
-          } catch (error) {
-            this.reject(error);
-          }
-        }
-      }
-
-      onTimeout() {}
-    })(this);
-
-    this.waiters.push(exit);
-
-    this.completed = exit.promise
-      .then(() => {
-        this.status = { type: 'completed' };
-      })
-      .catch(error => {
-        this.status = { type: 'errored', error };
-        throw error;
-      });
-  }
-
-  get isRunning(): boolean {
-    return this.status.type === 'running';
-  }
-
-  get isErrored(): boolean {
-    return this.status.type === 'errored';
   }
 
-  get isCompleted(): boolean {
-    return this.status.type === 'completed';
+  private async internalWait(timedOut?: Promise<void>): Promise<void> {
+    if (this.currentWaiter) {
+      throw new Error(`bug: only one wait at a time`);
+    }
+    try {
+      await Promise.race(
+        [
+          timedOut,
+          new Promise<void>(resolve => {
+            this.currentWaiter = resolve;
+          }),
+        ].filter(Boolean)
+      );
+    } finally {
+      this.currentWaiter = undefined;
+    }
   }
 
-  get recentOutput(): string {
-    return this.lines.join('\n');
+  private searchLines(output: string | RegExp): boolean {
+    while (this.nextWaitedLine < this.lines.length) {
+      let line = this.lines[this.nextWaitedLine++];
+      if (typeof output === 'string') {
+        if (output === line) {
+          return true;
+        }
+      } else {
+        if (output.test(line)) {
+          return true;
+        }
+      }
+    }
+    return false;
   }
 
-  async waitFor(output: string | RegExp, timeout?: number | null): Promise<void> {
-    const waiter = new OutputWaiter(this, output, timeout);
-
-    for (const line of this.lines) {
-      if (waiter.onOutputLine(line)) {
+  async waitFor(output: string | RegExp, timeout = DEFAULT_TIMEOUT): Promise<void> {
+    let timedOut = new Promise<void>((_resolve, reject) => {
+      setTimeout(() => {
+        let err = new Error(
+          'Timed out after ' +
+            timeout +
+            'ms before output "' +
+            output +
+            '" was found. ' +
+            'Output:\n\n' +
+            this.lines.join('\n')
+        );
+        reject(err);
+      }, timeout);
+    });
+    while (true) {
+      if (this.exitCode != null) {
+        throw new Error(
+          'Process exited with code ' +
+            this.exitCode +
+            ' before output "' +
+            output +
+            '" was found. ' +
+            'Output:\n\n' +
+            this.lines.join('\n')
+        );
+      }
+      if (this.searchLines(output)) {
         return;
       }
+      await this.internalWait(timedOut);
     }
-
-    this.waiters.push(waiter);
-    await waiter.promise;
-  }
-
-  clearOutput(): void {
-    this.lines = [];
   }
 
   async shutdown(): Promise<void> {
-    if (this.isErrored || this.isCompleted) {
+    if (this.exitCode != null) {
       return;
     }
 
@@ -223,7 +124,16 @@ class EmberCLI {
       }
     });
 
-    await this.completed;
+    await this.waitForExit();
+  }
+
+  async waitForExit(): Promise<number> {
+    while (true) {
+      if (this.exitCode != null) {
+        return this.exitCode;
+      }
+      await this.internalWait();
+    }
   }
 }
 
@@ -393,7 +303,6 @@ app.forEachScenario(scenario => {
       app = await scenario.prepare();
       server = EmberCLI.launch(['serve', '--port', '0'], { cwd: app.dir });
       await waitFor(/Serving on http:\/\/localhost:[0-9]+\//, DEFAULT_TIMEOUT * 2);
-      server.clearOutput();
     });
 
     hooks.afterEach(async () => {
@@ -410,7 +319,6 @@ app.forEachScenario(scenario => {
       await waitFor(/Build successful/);
 
       assert.true(await checkScripts(/js$/, originalContent), 'the file now exists');
-      server.clearOutput();
 
       const updatedContent = 'THREE IS A GREAT NUMBER TWO';
       assert.false(await checkScripts(/js$/, updatedContent), 'file has not been created yet');
@@ -455,7 +363,6 @@ app.forEachScenario(scenario => {
           import templateOnlyComponent from '@ember/component/template-only';
           export default templateOnlyComponent();
         `);
-        server.clearOutput();
 
         await appFile('app/components/hello-world.hbs').delete();
         await deleted('components/hello-world.hbs');
@@ -492,7 +399,6 @@ app.forEachScenario(scenario => {
         await assertRewrittenFile('components/hello-world.hbs').doesNotExist();
         await assertRewrittenFile('components/hello-world.js').doesNotExist();
         await assertRewrittenFile('tests/integration/hello-world-test.js').includesContent('<HelloWorld />');
-        server.clearOutput();
 
         await appFile('app/components/hello-world.js').write(d`
           import Component from '@glimmer/component';
@@ -506,11 +412,10 @@ app.forEachScenario(scenario => {
           export default class extends Component {}
         `);
         await assertRewrittenFile('tests/integration/hello-world-test.js').includesContent('<HelloWorld />');
-        server.clearOutput();
 
         let test = await EmberCLI.launch(['test', '--filter', 'hello-world'], { cwd: app.dir });
         await test.waitFor(/^not ok .+ Integration | hello-world: it renders/, DEFAULT_TIMEOUT * 2);
-        await assert.rejects(test.completed);
+        await assert.notStrictEqual(await test.waitForExit(), 0);
 
         await appFile('app/components/hello-world.hbs').write('hello world!');
         await added('components/hello-world.hbs');
@@ -525,7 +430,7 @@ app.forEachScenario(scenario => {
 
         test = await EmberCLI.launch(['test', '--filter', 'hello-world'], { cwd: app.dir });
         await test.waitFor(/^ok .+ Integration | hello-world: it renders/);
-        await test.completed;
+        await assert.strictEqual(await test.waitForExit(), 0);
       });
 
       test('Scenario 3: deleting a co-located template', async function () {
@@ -541,7 +446,6 @@ app.forEachScenario(scenario => {
           import templateOnlyComponent from '@ember/component/template-only';
           export default templateOnlyComponent();
         `);
-        server.clearOutput();
 
         await appFile('app/components/hello-world.js').write(d`
           import Component from '@glimmer/component';
@@ -555,7 +459,6 @@ app.forEachScenario(scenario => {
           import Component from '@glimmer/component';
           export default class extends Component {}
         `);
-        server.clearOutput();
 
         await appFile('app/components/hello-world.hbs').delete();
         await deleted('components/hello-world.hbs');
@@ -574,7 +477,6 @@ app.forEachScenario(scenario => {
         await appFile('app/components/hello-world.hbs').write('hello world!');
         await added('components/hello-world.hbs');
         await waitFor(/Build successful/);
-        server.clearOutput();
 
         await appFile('app/components/hello-world.js').write(d`
           import Component from '@glimmer/component';
@@ -582,7 +484,6 @@ app.forEachScenario(scenario => {
         `);
         await added('components/hello-world.js');
         await waitFor(/Build successful/);
-        server.clearOutput();
 
         await assertRewrittenFile('components/hello-world.hbs').hasContent('hello world!');
         await assertRewrittenFile('components/hello-world.js').hasContent(d`