Invisible BeginScrollView thumb

I have a GUI Area set up with a Scroll View and Buttons, but for some the reason the thumb on the Scroll View's Horizontal Scroll Bar doesn't show up. Here is my code:

 float buttonTop = 690.0f;
 Vector2 scrollPosition = Vector2.zero;
 
 void OnGUI()
     {
         GUILayout.BeginArea(new Rect(0.0f, buttonTop, Screen.width, Screen.height - buttonTop));
         scrollPosition = GUILayout.BeginScrollView(scrollPosition);
         GUILayout.BeginHorizontal();
         foreach (string clipName in animator.GetClipNames())
         {
             GUILayout.Button("Play " + clipName, GUILayout.MaxWidth(128.0f), GUILayout.Height((Screen.height - buttonTop) * 0.5f));
         }
         GUILayout.EndHorizontal();
         GUILayout.EndScrollView();
         GUILayout.EndArea();
     }

It compiles and runs just fine, and the Scroll View even works, but the thumb on the Scroll Bar is invisible. Any clues?