The Shield Push feat lets you push a target one square as part of a fighter's combat challenge, and combat challenge is an immediate interrupt you can take if the enemy shifts. So if they start a shift, the fighter can interrupt it and if they hit, push them 1 square. Does that invalidate the shift? If so, does it always invalidate the shift, or only if the target's intended destination is no longer within range of a 5 foot shift? Or can the target still take a 5 foot shift but pick a new destination square?