UI Toolkit version of the IMGUI DropDownButton

DropDownButton.cs

abstract class DropDownButton : BaseField<string>
{
   TextElement m_TextElement;
   VisualElement m_ArrowElement;
 
   public DropDownButton(string label)
       : base(label, null)
   {
       AddToClassList(EnumField.ussClassName);
       labelElement.AddToClassList(EnumField.labelUssClassName);
       visualInput.AddToClassList(EnumField.inputUssClassName);
 
       m_TextElement = new TextElement();
       m_TextElement.AddToClassList(EnumField.textUssClassName);
       m_TextElement.pickingMode = PickingMode.Ignore;
       visualInput.Add(m_TextElement);
 
       m_ArrowElement = new VisualElement();
       m_ArrowElement.AddToClassList(EnumField.arrowUssClassName);
       m_ArrowElement.pickingMode = PickingMode.Ignore;
       visualInput.Add(m_ArrowElement);
 
       var click = new Clickable(OnClick);
       visualInput.pickingMode = PickingMode.Position;
       visualInput.AddManipulator(click);
   }
   
   // Show your menu
   protected abstract void OnClick();
 
   public override void SetValueWithoutNotify(string newValue)
   {
       base.SetValueWithoutNotify(newValue);
       m_TextElement.text = newValue;
   }
}

visualInput is internal but can be replaced with a Query for the element with the classname inputUssClassName:

var visualInput = this.Q(className: BaseField<string>.inputUssClassName);

We also have EditorToolbarDropdown