Whisk together all the sauce ingredients until the sugar dissolves. Set aside.
Dice the chicken thighs into even 1.5-inch pieces and transfer to a large bowl. Don't pat them dry, as the natural surface moisture helps the cornstarch stick.
Season the diced chicken with salt and garlic powder, then add the cornstarch and coat evenly. If any areas remain uncoated, add 1-2 tablespoons more cornstarch as needed.
Heat vegetable oil in a large, heavy-bottomed pan over medium-high heat to 350°F. Fry the chicken in a single layer, working in small batches, for about 5-7 minutes total. Leave the chicken undisturbed until the bottom is golden and the crust has set before flipping. Cook until both sides are golden and crispy and the internal temperature reaches 165°F.
Transfer each batch to a wire rack to keep the coating crispy. Remove any browned bits from the oil and let it return to 350°F before frying the next batch.
Remove all but 1 teaspoon (5 ml) of oil from the pan and reduce the heat to medium. Add the onion, red bell pepper, and pineapple, and sauté for about 30-45 seconds until crisp-tender.
Whisk the sauce again, pour it into the pan, and simmer for about 30-45 seconds until thickened and glossy.
Add the fried chicken and toss until evenly coated in the sauce. Remove from the heat, garnish with green onions and toasted sesame seeds, and serve immediately.