Marinate the chicken: In a bowl, whisk together the pineapple juice, soy sauce, ketchup, brown sugar, sherry, vinegar, ginger, and garlic. Add chicken thighs to a large zip-top bag and pour in 2 cups of the marinade. Seal and refrigerate for 2-4 hours, reserving 1 cup of marinade for glazing.

Preheat the grill: Remove chicken from the refrigerator 30 minutes prior to grilling. Preheat your grill to medium (350°F–400°F). Pat chicken dry with paper towels and discard the marinade in the bag.

Grill the chicken: Oil the grill grates lightly. Place chicken on the grill and cook covered for 3 minutes on each side until grill marks form. Flip every 2 minutes, continuing to cook until the chicken's internal temperature hits 135°F.

Glaze and finish cooking: Brush the reserved marinade on the chicken and continue flipping every 1-2 minutes. Cook until an instant-read thermometer registers 165°F-175°F. Remove to a platter and let rest for 10 minutes.

Grill the pineapple: Increase grill temperature to medium-high (400°F–450°F). Place pineapple rings on the oiled grates and cook for 4 minutes, flipping once, until caramelized and bearing light grill marks. Serve alongside chicken, garnished with scallions.