i'm sure this is probably already known, but I just thought i'd post it here just incase, it's a glitch that can be a little niggly at times.
it only happens on OS X, not Windows. And i've tried it on 3 different Mac computers and they all do the same thing.
When scrolling the patcher window to the right, and then back to the left, this visual glitch is left behind, and the only way to get rid of it is to briefly make the auto hiding contraptions list appear which covers it then make the list hide again, which makes the glitch go away.
below are some screenshots i've taken which document the problem.