there's a race condition that Thread.start() doesn't imply the thread to get going right away.

So consider the thread running as soon as we create it. This allows the UI to show the progressive log icon right away without the user reloading back later

core/src/main/java/hudson/model/TaskThread.java

@@ -80,6 +80,7 @@ public abstract class TaskThread extends Thread {
         this.owner = owner;
         this.text = this.log = output.text;
         this.listener = output.listener;
+        this.isRunning = true;
     }
 
     public Reader readAll() throws IOException {