Output stderr of the underlying script

src/Screen.php

@@ -75,6 +75,9 @@ public function subscribeToProcessOutput(Process $process): void
         $process->stdout->on('data', static function ($data) {
             echo $data;
         });
+        $process->stderr->on('data', static function ($data) {
+            echo $data;
+        });
     }
 
     public function showSpinner(LoopInterface $loop): void

tests/Helper/Filesystem.php

@@ -24,6 +24,14 @@ public static function createHelloWorldPHPFile(): string
         return $name;
     }
 
+    public static function createStdErrorPHPFile(): string
+    {
+        $name = self::FIXTURES_DIR . 'test.php';
+        self::createFile($name, '<?php fwrite(STDERR, "Some error");');
+
+        return $name;
+    }
+
     public static function createHelloWorldPHPFileWithSignalsHandling(): string
     {
         $name = self::FIXTURES_DIR . 'test.php';

tests/RunScriptTest.php

@@ -20,4 +20,16 @@ public function it_runs_a_php_script(): void
         $this->assertStringContainsString("starting `php $scriptToRun`", $output);
         $this->assertStringContainsString('Hello, world', $output);
     }
+
+    /** @test */
+    public function it_outputs_the_script_stderr(): void
+    {
+        $scriptToRun = Filesystem::createStdErrorPHPFile();
+        $watcher = (new WatcherRunner)->run($scriptToRun);
+
+        $this->wait();
+        $output = $watcher->getOutput();
+
+        $this->assertStringContainsString('Some error', $output);
+    }
 }