IN A LARGE BOWL, COMBINE PINEAPPLE JUICE, CHICKEN BROTH, BROWN SUGAR, SOY SAUCE, AND GARLIC.
PLACE CHICKEN THIGHS INTO A 6-QT SLOW COOKER. STIR IN PINEAPPLE JUICE MIXTURE. TOP WITH PINEAPPLE CHUNKS AND RESERVED JUICES.
COVER AND COOK ON LOW HEAT FOR 5-6 HOURS OR HIGH FOR 2-3 HOURS, BASTING EVERY FEW HOURS.
IN A SMALL BOWL, WHISK TOGETHER 1/4 CUP WATER AND CORNSTARCH.
STIR IN THE MIXTURE INTO THE SLOW COOKER. COVER AND COOK ON HIGH HEAT FOR AN ADDITIONAL 30 MINUTES, OR UNTIL SAUCE IS THICKENED.
OPTIONAL: PREHEAT OVEN TO BROIL. PLACE CHICKEN THIGHS ONTO A BAKING SHEET, SKIN SIDE UP, AND BROIL UNTIL CRISP, ABOUT 3-4 MINUTES.
SERVE CHICKEN IMMEDIATELY, GARNISHED WITH SESAME SEEDS AND PARSLEY, IF DESIRED.