CustomContextMenu

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

public class CustomWindow : EditorWindow
{
    private bool showContextMenu;
    private Vector2 mousePosition;
    private List<string> menuItems;
    private GUISkin contextMenuSkin;
    private GUIStyle buttonStyle;
    private float buttonHeight;
    private Vector2 scrollPosition;
    private string searchString;
    private float topOffset = 25f;

    // Add a menu to the Unity Editor which when selected
    // will call this static method
    [MenuItem("My Window/Show")]
    public static void ShowWindow()
    {
        // Create a new instance of the window
        CustomWindow window = GetWindow<CustomWindow>();
        // This isn't required it just prevents the window
        // from being resized smaller than 800x600. The same
        // can be done for the maxSize
        window.minSize = new Vector2(800, 600);
        // Show the newly created window
        window.Show();
    }

    private void OnGUI()
    {
        // Check if the skin has been loaded, if not load it
        if (contextMenuSkin == null)
        {
            contextMenuSkin = AssetDatabase.LoadAssetAtPath<GUISkin>("Assets/ContextMenuSkin.guiskin");
            buttonStyle = contextMenuSkin.GetStyle("contextMenuButton");
        }

        // Specifies where and how big to draw the menu
        Rect clickedArea = new Rect(mousePosition.x, mousePosition.y, 120, (6 * buttonHeight) + 5 + topOffset);

        // Get the current event being processed
        Event current = Event.current;

        // Check to see if the the mouse was clicked
        if (current.type == EventType.MouseDown)
        {
            // Check if it was the right mouse button
            if (current.button == 1)
            {
                if (!showContextMenu)
                {
                    // If we aren't currently showing the context
                    // menu, get the current mouse position and
                    // set showContextMenu to true
                    mousePosition = current.mousePosition;
                    showContextMenu = true;
                }
                else
                {
                    // Hide the context menu
                    showContextMenu = false;
                }

                // Use the event causing other GUI elements to ignore it
                current.Use();
            }
            else
            {
                // If we click outside the menu we want to hide it
                if (!clickedArea.Contains(current.mousePosition))
                    showContextMenu = false;
            }
        }

        // Initialize the list of menu items
        menuItems = new List<string>();

        // Add simple strings to the list
        for(int i = 0; i < 6; i++)
            menuItems.Add("Menu " + i);

        buttonHeight = buttonStyle.CalcSize(new GUIContent(menuItems[0])).y;

        // Check the flag we set above to see if we can display the menu
        if (showContextMenu)
        {
            // Specify where we want to draw the controls contained inside
            // this area
            GUILayout.BeginArea(clickedArea);
            {
                // Use a Scroll View in case we have a lot of menu items.  We don't to
                // display a huge menu
                scrollPosition = GUILayout.BeginScrollView(scrollPosition, contextMenuSkin.scrollView);
                {
                    // Use a text field but style it as a search field.  Due to a type in Unity,
                    // make sure you take note of the misspelling of the GUI style
                    searchString = GUILayout.TextField(searchString, GUI.skin.FindStyle("ToolbarSeachTextField")) ?? string.Empty;

                    List<string> filteredMenuItems = menuItems.Where(menuItem => menuItem.Contains(searchString)).ToList();

                    // Loop through all the menu items
                    for (int i = 0; i < filteredMenuItems.Count; i++)
                    {
                        // Draw a button for each menu item
                        if (GUI.Button(new Rect(0, i * buttonHeight + topOffset, 105, buttonHeight), filteredMenuItems[i], buttonStyle))
                        {
                            showContextMenu = false;

                            // Button code goes here
                        }
                    }
                }
                GUILayout.EndScrollView();
            }
            GUILayout.EndArea();
        }
    }
}