Who looked down from a window and despised David for dancing before the LORD?

Answer: Michal

Saul's daughter Michal mocked David. He said he was celebrating before the LORD, and Michal had no children to the day of her death.

“As the ark of the LORD was entering the City of David, Saul’s daughter Michal looked down from a window and saw King David leaping and dancing before the LORD, and she despised him in her heart.”— 2 Samuel 6:16 (BSB)