gee, i feel so smart today.
i finally figured out what those bars do!
they are a visible range zoom/limiter, and scroll bar.
makes it a bit easier to navigate and fine tune breakpoints. still, why the visible length of segments is displayed in like a logarithmic fashion beats me.
what this means is that 3/4's of the total length used in a whole 10 seconds is only covering 1 second in time, where the following 9 seconds occur in that last 1/4 of the availble visible range.