Vertical green line on timeline

I suddenly have a vertical green line on the left side of the image throughout my timeline. It happens for all sequences and it happens when I create a new project and import other source footage. 

I've tried rendering with Mercury Playback Engine GPU (Metal), with Mercury Playback Engine GPU Acceleration (Open CL) and Mercury Playback ENgine Software Only, all with the same results. In the new—test—project I imported a finished video without any effects to render, and the result was the same.

I've attached an image to show the problem.

The good news is that the line doesn't seem to appear in exports.

Any idea what's up?