The project folder is not "fixed". Most often than not, when opening a lot of folders so that the scrollbar is outside the visibile bounds, collapsing some of them again moves the project folders view completely. I think the correct behaviour would be that anything above the sourcetree item I am folding stays exactly where it is. It looks like the whole scrollbar is repositioned. This is often confusing and I have to spend a second or two finding my way again.
Customer support service by UserEcho