Piano Roll Review - Cannot select anything

Every once in a while in X1, I find that the Piano Roll View no longer lets me select notes.  I can delete them by right-clicking on them, but I cannot select them individually or by dragging around them; the drag-box never appears.

Restarting X1 makes the problem go away.  However, I'd like to find a fix which doesn't require a restart.

Any ideas?

Could this be related to the fact that I'm using Sonar Plus?