jit.pwindow artifacts when using jit.mgraphics

I'm trying to create a simple sine wave animation with a head and a tail for an online course I am taking. I'm using a jit.mgraphics object to write to a jit.pwindow. I naively thought that re-stroking in white over a location where I had previously stroked in black would cleanly clear the previous stroke, but I'm seeing weird artifacts (outlines) wherever I try to draw over where I've previously drawn. I thought this might be related to anti-aliasing, but fsaa is turned off in the pwindow. Any insight into why this happens (and more importantly, how to fix it)?