StaggeredPanel left/right keyboard navigation doesn't work

Describe the bug

When using the keyboard to navigate a StaggeredPanel, you can only use up/down arrow keys to navigate between items. I'd expect left/right to work as well.

Steps to Reproduce

Steps to reproduce the behavior:

1. Open StaggeredPanel sample in sample app
2. Click on an item in the list
3. Use up/down arrows to navigate between items, to say item 2
4. Try and hit right arrow to go to item 3, nothing happens

Expected behavior

Focus to move to item 3 from 2 when right arrow key is pressed.

Environment:

Insider Build - Sample App