Quote from vbgamestudio on 22 August 2023, 11:27

As of version 2023.1 an icon seems to have been removed from the Unity icons list.

If you hide the content by clicking on Hide main setting or Hide options it will no longer be possible to go back.

The issue will be fixed in the next version we are about to release.

In the meantime you can solve it like this:

In the Editor folder open Setup25dTKEditor.cs and MenuOptEditor.cs. You will find the code to change at the top of both files.

GUILayout.BeginHorizontal();
if (nav.isHideOptionSettings) { // or if (nav.isHideMainSettings) 
if (GUILayout.Button(EditorGUIUtility.IconContent("winbtn_win_max_h"), lblCol, GUILayout.ExpandWidth(false))) {
nav.isHideOptionSettings = nav.treeState[1] = false;
}
EditorGUILayout.LabelField("Show options", lblCol);
}
else {
if (GUILayout.Button(EditorGUIUtility.IconContent("winbtn_win_min_h"), lblCol, GUILayout.ExpandWidth(false))) {
nav.isHideOptionSettings = nav.treeState[1] = true;
}
EditorGUILayout.LabelField("Hide options", lblCol);
}
GUILayout.EndHorizontal();

Change "winbtn_win_max_h" to "winbtn_win_min_h".