OnBecameVisible and OnBecameInvisible are in the Unity docs as methods of the Renderer and MonoBehaviour classes. It turns out that they work for EditorWindows, too!

They get called when a docked window changes visibility; meaning that when a docked window is hidden by a sibling window within that dock, OnBecameInvisible gets called. Similarly, when the window becomes visible again, of course, OnBecameVisible is called.

using UnityEditor;
using UnityEngine;


public class OnBecameVisibleEditorWindow : EditorWindow
{
	private void OnBecameVisible()
	{
		Debug.Log("Window is visible");
	}

	private void OnBecameInvisible()
	{
		Debug.Log("Window is invisible");
	}


	[MenuItem("Tools/OnBecameVisible Window")]
	public static void Init_ToolsMenu()
	{
		EditorWindow window = EditorWindow.GetWindow<OnBecameVisibleEditorWindow>();
		window.title = "OnBecameVisible";
		window.Show();
	}
}

There is a little gotcha, though. It seems that both methods get called when docking and undocking the window. Not sure why, really. It’s just another Unity quirk!