It originally inherited from ChangeEvent!String, but now does from ChangeEventBase as of January 3, 2025. This shouldn't break anything if you used it through either its own name SelectionChangedEvent or through the base Event, only if you specifically used ChangeEvent!string - those handlers may now get null or fail to be called. If you did do this, just change it to generic Event, as stringValue and intValue are already there.
This event is fired when the selection changes. Both Event.stringValue and Event.intValue are filled in - stringValue is the text in the selection and intValue is the index of the selection. If the combo box allows multiple selection, these values will include only one of the selected items - for those, you should loop through the values and check their selected flag instead.
(I know that sucks, but it is how it is right now.)