Unity3D: Master-detail layout in custom editor.

CustomEditorWindow.cs

using UnityEngine;
using UnityEditor;
using System.Collections;
using Opendream.Layouts;

namespace Opendream {

	public partial class CustomEditorWindow : EditorWindow {

		[MenuItem("Window/Custom Editor", false, 2)]
		public static void OpenCustomEditorWindow() {
			EditorWindow.GetWindow<CustomEditorWindow>(false, "Custom");
		}
		
		private MasterDetailLayout mdLayout;
		
		private void OnGUI() {
			if (mdLayout == null) {
				mdLayout = new MasterDetailLayout(this);
				mdLayout.onDrawList += OnDrawList;
				mdLayout.onDrawDetail += OnDrawDetail;
			}
		}
		
		private void OnDrawList() {
			// Do your stuff.
		}
		
		private void OnDrawDetail() {
			// Do your stuff.
		}
	
	}

}

MasterDetailLayout.cs

using UnityEngine;
using UnityEditor;

namespace Opendream.Layouts {
	
	public class MasterDetailLayout {

		public delegate void OnDrawList();

		public delegate void OnDrawDetail();

		/// <summary>
		/// The on draw list delegate.
		/// </summary>
		public OnDrawList onDrawList;

		/// <summary>
		/// The on draw detail delegate.
		/// </summary>
		public OnDrawDetail onDrawDetail;

		/// <summary>
		/// The list pane width percentage.
		/// </summary>
		private float listPaneWidthPercentage = 0.3f;

		/// <summary>
		/// The width of the list pane.
		/// </summary>
		private float listPaneWidth;

		/// <summary>
		/// The minimum width of the list pane.
		/// </summary>
		private float MinListPaneWidth = 150f;

		/// <summary>
		/// The width of the splitter.
		/// </summary>
		private float splitterWidth = 5f;

		/// <summary>
		/// The list pane position.
		/// </summary>
		private Vector2 listPanePosition;

		/// <summary>
		/// The resizing.
		/// </summary>
		private bool resizing = false;

		/// <summary>
		/// The list scroll position.
		/// </summary>
		private Vector2 listScrollPosition;

		/// <summary>
		/// The detail scroll position.
		/// </summary>
		private Vector2 detailScrollPosition;

		/// <summary>
		/// The window.
		/// </summary>
		public EditorWindow window;

		/// <summary>
		/// Initializes a new instance of the <see cref="Opendream.Platformer2D.Backend.MasterDetailLayout"/> class.
		/// </summary>
		public MasterDetailLayout() {}

		/// <summary>
		/// Initializes a new instance of the <see cref="Opendream.Platformer2D.Backend.MasterDetailLayout"/> class.
		/// </summary>
		/// <param name="window">Window.</param>
		public MasterDetailLayout(EditorWindow window) {
			this.window = window;
		}

		/// <summary>
		/// Draws the layout.
		/// </summary>
		/// <returns>The layout.</returns>
		public void DrawLayout() {
			listPaneWidth = Mathf.Clamp(window.position.width * listPaneWidthPercentage, MinListPaneWidth, window.position.width);

			GUILayout.BeginHorizontal();
			DrawListPane();
			DrawSplitter();
			DrawDetailPane();
			GUILayout.EndHorizontal();
		}

		/// <summary>
		/// Draws the list pane.
		/// </summary>
		private void DrawListPane() {
			listScrollPosition = EditorGUILayout.BeginScrollView(listScrollPosition, GUILayout.Width(listPaneWidth));

			if (onDrawList != null) {
				onDrawList();
			}

			EditorGUILayout.Space();
			EditorGUILayout.Space();

			EditorGUILayout.EndScrollView();

			Rect rect = GUILayoutUtility.GetLastRect();
			Rect borderRect = new Rect(rect.x + rect.width, rect.yMin, 1, rect.height);
			GUIStyle borderStyle = new GUIStyle();

			Texture2D backgroundTexture = new Texture2D(1, 1, TextureFormat.RGBA32, false);
			backgroundTexture.SetPixel(0, 0, CustomGUIStyle.UnfocusedColor);
			backgroundTexture.Apply();
			backgroundTexture.hideFlags = HideFlags.HideAndDontSave;
			borderStyle.normal.background = backgroundTexture;
			GUI.Box(borderRect, "", borderStyle);
		}

		/// <summary>
		/// Draws the splitter.
		/// </summary>
		private void DrawSplitter() {
			Rect rect = GUILayoutUtility.GetLastRect();
			Rect splitterRect = new Rect(rect.x + rect.width, rect.yMin, splitterWidth, rect.height);
			GUI.Box(splitterRect, "", GUIStyle.none);
			EditorGUIUtility.AddCursorRect(splitterRect, MouseCursor.SplitResizeLeftRight);
			
			if (Event.current != null) {
				switch (Event.current.type) {
				case EventType.MouseDown:
					if (splitterRect.Contains(Event.current.mousePosition)) {
						resizing = true;
					}
					break;
				case EventType.MouseDrag:
					if (resizing) {
						listPaneWidth = Mathf.Clamp(Event.current.mousePosition.x, MinListPaneWidth, window.position.width);
						listPaneWidthPercentage = listPaneWidth / window.position.width;
						window.Repaint();
					}
					break;
				case EventType.MouseUp:
					resizing = false;
					break;
				}
			}
		}

		/// <summary>
		/// Draws the detail pane.
		/// </summary>
		private void DrawDetailPane() {
			detailScrollPosition = GUILayout.BeginScrollView(detailScrollPosition, GUILayout.ExpandWidth(true));

			if (onDrawDetail != null) {
				onDrawDetail();
			}

			GUILayout.EndScrollView();
		}

	}

}