A drag and droppable 'scratchpad' for unity objects, useful as a palette or a quick select window.

AssetScratchpad.cs

using UnityEditor;
using UnityEngine;
using System.Collections.Generic;

public class AssetScratchpad : EditorWindow
{
  private List<Object> _referenceList = new();
  private Vector2 _scrollPosition;

  [MenuItem("Window/Asset Scratchpad")]
  public static void ShowWindow()
  {
    EditorWindow.GetWindow<AssetScratchpad>("Asset Scratchpad");
  }

  private void OnGUI()
  {
    // Draw drop area
    Rect dropArea = GUILayoutUtility.GetRect(0.0f, 50.0f, GUILayout.ExpandWidth(true));
    var styleBox = new GUIStyle(GUI.skin.box);
    styleBox.normal.textColor = Color.white;
    styleBox.hover.textColor = Color.white;
    styleBox.alignment = TextAnchor.MiddleCenter;
    GUI.Box(dropArea, "Drop objects here", styleBox);

    // Clear button
    if (GUI.Button(new Rect(0, 0, 80, 20), "Clear"))
    {
      _referenceList.Clear();
    }

    // Contain list in a scrollview
    _scrollPosition = GUILayout.BeginScrollView(_scrollPosition);

    for (int i = 0; i < _referenceList.Count; i++)
    {
      if (_referenceList[i] == null)
      {
        _referenceList.RemoveAt(i);
        continue;
      }

      EditorGUILayout.BeginHorizontal();
      DrawDraggableObjectField(_referenceList[i], i);
      if (GUILayout.Button("Remove", GUILayout.MaxWidth(80)))
      {
        _referenceList.RemoveAt(i);
      }
      EditorGUILayout.EndHorizontal();
    }

    GUILayout.EndScrollView();

    // Handle events
    Event evt = Event.current;
    switch (evt.type)
    {
      case EventType.DragUpdated:
      case EventType.DragPerform:
        DragAndDrop.visualMode = DragAndDropVisualMode.Copy;

        if (evt.type == EventType.DragPerform)
        {
          DragAndDrop.AcceptDrag();

          foreach (Object obj in DragAndDrop.objectReferences)
          {
            if (!_referenceList.Contains(obj))
              _referenceList.Add(obj);
          }
        }
        Event.current.Use();
        break;
    }
  }

  private void DrawDraggableObjectField(Object obj, int index)
  {
    Rect rect = GUILayoutUtility.GetRect(0, EditorGUIUtility.singleLineHeight);

    // Get the content (icon and text) for the object
    GUIContent content = EditorGUIUtility.ObjectContent(obj, typeof(Object));

    // Draw the icon
    GUI.Label(new Rect(rect.x, rect.y + 2, 20, EditorGUIUtility.singleLineHeight), content.image);

    // Draw the object field
    EditorGUI.LabelField(new Rect(rect.x + 25, rect.y + 2, rect.width - 25, EditorGUIUtility.singleLineHeight),
        new GUIContent(obj.name, "Drag to another field"), EditorStyles.objectField);

    Event evt = Event.current;
    switch (evt.type)
    {
      // Allow dragging from field
      case EventType.MouseDrag:
        if (rect.Contains(evt.mousePosition))
        {
          DragAndDrop.PrepareStartDrag();
          DragAndDrop.objectReferences = new Object[] { obj };
          DragAndDrop.StartDrag(obj.name);
          Event.current.Use();
        }
        break;

      // Ping objects on click
      case EventType.MouseUp:
        if (rect.Contains(evt.mousePosition) && evt.button == 0)
        {
          EditorGUIUtility.PingObject(obj);
        }
        else if (rect.Contains(evt.mousePosition) && evt.button == 1)
        {
          EditorUtility.OpenPropertyEditor(obj);
        }
        break;
    }
  }
}