code: @Override
      protected IAbortable removeThread(IAbortable thread) {
          super.removeThread(thread);
  
          if (!hasActiveThreads()) {
              showBusy(null);
          }
  
          return thread;
      }
docs: Remove a thread from the active list. Clears the busy state if this was the last active
  thread.
  
  @param thread Thread to remove.
  @return The thread that was removed.