If your UI is based on multi selection (and Nautilus definitely is) you should not register to whenSelectedIndexChanged:/whenSelectedItemChanged:, but only the plural version.

Ok, I get that. 

Still, when you create a NewList you have to provide a setIndexSelector, for example in the CategoryWidget I am doing:

categoriesList := NewList

on: self 

getItemsSelector: #getCategories 

setIndexSelector: #selectedCategoryIndex:

getDisplaySelector: #categoryWrapper:.

So, every time you click on the categories list (both selecting multiple categories or not) the CategoryWidget receives the #selectedCategoryIndex: message.

The current selectedCategoryIndex: message reads like this:

selectedCategoryIndex: anInteger

| anObject |

anObject := self getCategories at: anInteger ifAbsent: [ nil ].

self model selectedCategory: anObject.

self changed: #selectedCategoryIndex.

self model categorySelectionChanged.

self model changed: #currentHistoryIndex.

Also the NewList is in the list of dependents of the CategoryWidget, so it also gets the #update: message

Is this right?