In my experience, an always visible folder pane is just taking up precious screen real estate 90% of the time.
My idea is to have the folder pane collapse to the left, appear when the mouse is positions on the left border for more than half a sec. You can also probably have an options bar at the bottom of the folder pane where a user would be able to set various attributes like "show after X seconds mouse is on left border","disable auto hide", etc.
This would really help when I have large subtrees of folders, and I'd have to resize the folder pane all the way to the right everytime I wanted to select a folder on the lower levels of the tree, and push it back to the left when I wanted to get back to the code.
Customer support service by UserEcho