What is this black shadow that follows the mouse?

[gmp]

What is this black shadow that follows the mouse in the clip pane of the track view. This behavior has been around for many months or so. Same in CbB.

 

Look at the black shadow in bar 1. Bar 2 to the end has the typical timeline selection shadow. I included that so no one thinks it's that.

When this happens, it follows my mouse movement. I'm not even sure how or why it happens. It's hard to stop it from doing this when it starts. Has anyone seen this? When this happens, when I move my moue across the screen, the shadow get's longer until I stop moving my mouse. It only is seen in the clip pane 

Edited August 7 by gmp