Depending on the way your brush and pole is set up you should be able to adjust the angle of your brush to get over the obstruction and still have the brush flat on the glass. It will mean going further out away from the wall but it can still easily be done.
You may find it difficult if it is a very large flat roof, and the window is very low down near the roof, but these are very few and far between, I have only ever came across this once before.
Peter