How to make poster grip stop when it reaches the top or bottom of the list?

Ok so since posterGrid is the child of ArrayGrid you can use horizFocusAnimationStyle and vertFocusAnimationStyle. In your case you would probably want vertFocuseAnimation to be either floatingFocus or fixedFocus
https://developer.roku.com/en-gb/docs/references/scenegraph/abstract-nodes/arraygrid.md 
hope this helps.