I think it might be helpful if you were to explain why you chose to cast sender, which is an AutoSuggestBox, to an AutoSuggestBox on line 48 of MainPage.xaml.cs. As far as I can see it was done only to make lines 49 & 50 (arguably) a little more readable. I rewrote your code as
@mbensch: Thanks, your explanation of the Grid-first vs StackPanel-first approach makes perfect sense. I too started with Grid and wondered what the performance/real-world differences would be. I can see now how Bob's approach lends itself to easier data binding.