We are interested in the activity. It does not matter whether something has been finished or not. In this example, the activity (painting the ceiling) has not been finished.
The ceiling was white. Now it is blue. She has painted the ceiling. Has painted is the present perfect simple.
Here, the important thing is that something has been finished. ‘Has painted’ is a completed action. We are interested in the result of the activity (the painted ceiling), not in the activity itself.
My hands are very dirty. I’ve been repairing the car.