EditorApplication.focusChanged や isFocused でエディタがフォーカスされているか確認できる

EditorApplication.focusChanged の使用例

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class Example
{
    static Example()
    {
        EditorApplication.focusChanged += isFocused =>
        {
            Debug.Log( isFocused );
        };
    }
}

EditorApplication.isFocused の使用例

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public static class Example
{
    static Example()
    {
        EditorApplication.update += () =>
        {
            Debug.Log( EditorApplication.isFocused );
        };
    }
}

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