When you have multiple documents opened e.g.

Doc1, Doc2, Doc3, Doc4

and Doc4 IsActive and then you close Doc4 (still as active document), new active document will be Doc1 (or any document that is on index 0;

From my point of view i expect, to Doc3 will be selected (or any previous document).

When i looked into code, i noticed that

LayoutElement.cs 76 -> is raising OnParentChanging in Parent setter with new value Null
LayoutContent.cs 692 -> is setting IsSelected property to false for old document
LayoutContent.cs 154 -> setting SelectedContentIndex on parentSelector to -1

and then when AutoFixSelectedContent is called, correct SelectedContentIndex is never set

You can try this in yout TestApp simply by adding more than 3 documents, then set last to active and then close it