Distraction-Free Mode has bug?

Excited to see a distraction-free mode at last.  Upon using it, I notice that there is a large swatch of space on the left hand side of the screen (like one big empty column).  At first, I thought it was due to where the "Project/Structure" panels used to be.  But even closing those panels and then entering distraction-free mode still leaves a large empty column on the left of the UI.  Is this a bug?  Seems so.