Just tried that, but it doesn't work as I expect. I'm still sometimes missing SelectedItemsChanged events. I'm able to produce the result I want only if I attach event handlers to both SelectedItemsChanged and RecordActivated events. The RecordActivated event always tells me when a new record was selected and fortunately so far it is followed by a SelectedItemsChanged event when multiple items are selected. Since I'm just enabling/disabling actions on the number of selected items I'm happy to consume both events as long as the SelectedItemsChanged event is always fired after the RecordActivated event.

Activation and selection are not the same thing. RecordActivated tells you when a record was activated. When a record is active it is the focused item but the active record does not have to be selected. This is similar to how you can use the listview (e.g. in Windows Explorer) select one record but then navigate to another record without changing the selection (e.g. hold down ctrl and use the arrow keys) or you can deselect a record (or similarly a listview item) by holding the ctrl key when clicking on it.