TagProvider.cs

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using Unity.QuickSearch;
using UnityEngine;
using System;
using UnityEditor;
using UnityEditorInternal;

public class TagProvider : MonoBehaviour
{
    internal static string type = "tag";
    internal static string displayName = "Tag";

    [SearchItemProvider]
    internal static SearchProvider CreateProvider()
    {
        return new SearchProvider(type, displayName)
        {
            filterId = "tag:",
            fetchItems = (context, items, provider) =>
            {
                if(InternalEditorUtility.tags.Contains(context.searchQuery))
                    items.AddRange(
                        FindObjectsOfType(typeof(GameObject)).Cast<GameObject>()
                            .Where(go => go.CompareTag(context.searchQuery))
                            .Select(go =>
                            {
                                var id = go.GetInstanceID().ToString();
                                return provider.CreateItem(id, label:go.name, data:go);
                            })
                );
            },


            fetchThumbnail = (item, context) =>
            {
                if (item.thumbnail)
                    return item.thumbnail;

                var obj = ObjectFromItem(item);
                if (obj != null)
                {
                    item.thumbnail = PrefabUtility.GetIconForGameObject(obj);
                    if (item.thumbnail)
                        return item.thumbnail;
                    item.thumbnail = EditorGUIUtility.ObjectContent(obj, obj.GetType()).image as Texture2D;
                }

                return item.thumbnail;
            },

            startDrag = (item, context) =>
            {
                var obj = ObjectFromItem(item);
                if (obj != null)
                {
                    DragAndDrop.PrepareStartDrag();
                    DragAndDrop.objectReferences = new[] { obj };
                    DragAndDrop.StartDrag("Drag scene object");
                }
            },

            isItemValid = item => ObjectFromItem(item) != null
        };
    }


    [SearchActionsProvider]
    internal static IEnumerable<SearchAction> ActionHandlers()
    {
        return new SearchAction[]
        {
                    new SearchAction(type, "select", null, "Select object in scene...") {
                        handler = (item, context) =>
                        {
                            var obj = ObjectFromItem(item);
                            if (obj != null)
                            {
                                Selection.activeGameObject = obj;
                                EditorGUIUtility.PingObject(obj);
                                SceneView.lastActiveSceneView.FrameSelected();
                            }
                        }
                    }
        };
    }

    private static GameObject ObjectFromItem(SearchItem item)
    {
        var instanceID = Convert.ToInt32(item.id);
        var obj = EditorUtility.InstanceIDToObject(instanceID) as GameObject;
        return obj;
    }
}