WPF ListBox选择项目，在失去焦点时保持选中状态

I have a parent ListBox which is made of two listboxes, and a child listbox, so 3 listboxes in total. The Item source for the first two are just two separate collections, these collections contain inner collection which then become an item source for the third listbox. If I select second item then the child listbox is populated again, if I go to the second parent listbox and chose something then the child listbox is populated. The problem is that if at this point I go back to the first listbox, and try to select an item that was previously selected, nothing happens, while im expecting it to populate the child listbox again. I have a parent ListBox which is made of two li