What is the ruling on the prayer of a woman who intentionally uncovers a part of her hijab from the front, and is there a consensus among scholars that the prayer of one who intentionally uncovers a part of their 'awrah during it is invalid?

It is not permissible for a woman to intentionally uncover any part of her hair, even a small amount, during prayer. This holds true even according to those who say that covering the 'awrah is not a condition but an obligation; they explicitly state that if she uncovers a portion of her hair, she must repeat the prayer within its prescribed time. There is no disagreement regarding the obligation to cover the 'awrah during prayer. However, the view that uncovering the 'awrah, even a small amount, invalidates the prayer is not a consensus. Similarly, the fundamental condition of covering it for prayer is not a complete consensus. The preponderant view is that it is a condition for the validity of the prayer.