I think it's a bug in the way sonar handles the vertical line/cursor while scrubbing.
Sonar does a complete redisplay of the pane when you let go of the mouse which tells me they took the easy way out.
I get the mess whether the controller pane is visible or not.
Trying this out just now, I've become permanently stuck in scrub mode. The mouse will only scrub, whether I hold the j key or not, and the mouse icon is the speaker.
Clicking outside the pane does not fix the problem. If I insert a new track and bring up the PRV for it, I'm still stuck in scrub mode for the new track.
Restarting Sonar fixed it.
X2b please.