On*GUI should only switch on EventType and call methods to reduce code complexity

ExampleEditorSwitchOnEventType.cs

using CodeSmile.UnityEditor;
using System;
using UnityEditor;
using UnityEngine;
using UnityEngine.UIElements;

namespace CodeSmile.Tile.UnityEditor
{
	[CustomEditor(typeof(TileWorld))]
	public class TileWorldDrawer : Editor
	{
		private Vector3 m_CursorWorldPosition;
		private Vector3 m_CursorGridPosition;
		private bool m_LeftMouseButtonDown;

		private void OnSceneGUI()
		{
			switch (Event.current.type)
			{
				case EventType.MouseMove:
					break;
				case EventType.MouseDown:
					if (m_InputState.IsButtonDown(MouseButton.LeftMouse))
						Event.current.Use();
					break;
				case EventType.MouseDrag:
					UpdateTileCursorPosition(Event.current.mousePosition);
					TryDrawBrush();
					DrawTileCursor();
					if (m_InputState.IsButtonDown(MouseButton.LeftMouse))
						Event.current.Use();
					break;
				case EventType.ScrollWheel:
					break;
				case EventType.Repaint:
					UpdateTileCursorPosition(Event.current.mousePosition);
					DrawTileCursor();
					break;
				case EventType.Layout:
					AddDefaultControl();
					break;
				case EventType.DragUpdated:
					Debug.Log("drag update");
					break;
				case EventType.DragPerform:
					Debug.Log("drag perform");
					break;
				case EventType.DragExited:
					Debug.Log("drag exit");
					break;
				case EventType.Ignore:
					break;
				case EventType.Used:
					break;
				case EventType.ValidateCommand:
					break;
				case EventType.ExecuteCommand:
					break;
				case EventType.ContextClick:
					Debug.Log("context click");
					break;
				case EventType.MouseEnterWindow:
					break;
				case EventType.MouseLeaveWindow:
					break;
				case EventType.TouchMove:
					break;
				case EventType.TouchEnter:
					break;
				case EventType.TouchLeave:
					break;
				case EventType.TouchStationary:
					break;
			}
			}
		}

		private void TryDrawBrush()
		{
			if (m_LeftMouseButtonDown)
			{
				var tileWorld = (TileWorld)target;
				tileWorld.DrawTile(m_CursorGridPosition);
			}
		}

		private void UpdateTileCursorPosition(Vector2 mousePos)
		{
			var ray = HandleUtility.GUIPointToWorldRay(mousePos);
			if (Ray.IntersectsVirtualPlane(ray, out m_CursorWorldPosition))
			{
				var gridSize = ((TileWorld)target).Grid.Size;
				m_CursorGridPosition = HandlesExt.SnapPointToGrid(m_CursorWorldPosition, gridSize);
			}
		}

		private void DrawTileCursor()
		{
			var tileWorld = (TileWorld)target;
			var gridSize = ((TileWorld)target).Grid.Size;
			var gridPos = m_CursorGridPosition;
			var renderPos = Vector3.zero;

			switch (tileWorld.TilePivot)
			{
				case TilePivot.Center:
					renderPos = new Vector3(gridPos.x * gridSize.x + gridSize.x * .5f,
						gridPos.y * gridSize.y + gridSize.y * .5f,
						gridPos.z * gridSize.z + gridSize.z * .5f);
					break;
				default:
					throw new ArgumentOutOfRangeException();
			}

			Handles.DrawWireCube(renderPos, gridSize);
		}

		private void AddDefaultControl()
		{
			var controlId = GUIUtility.GetControlID(GetHashCode(), FocusType.Passive);
			HandleUtility.AddDefaultControl(controlId);
		}
	}
}