、。
She sat staring out the window.
Literal
She [topic-は] sit [then-て], window [genitive-の] outside [object-を] fixedly was-watching.
じっと is a 擬態語 (state mimetic) for being still and intent — fixed gaze, motionless waiting, focused attention. Pairs naturally with 見る (じっと見る 'stare') and する (じっとしている 'stay still'). The ~ていた past progressive captures the sustained, unbroken nature of her watching. The を on 窓の外 marks the path/area of perception — Japanese uses を to mark the surface gazed across, parallel to 道を歩く ('walk along the road').