Writing MIDI in step sequencer Addictive Drums and a White Line appears in the MIDI track

SPLAT, HP Envy 7 - don't know the specs but everything is updated, plenty of memory, PC is almost 2 years old.
Projects with drums - I always start there. So, clicked on something by mistake because suddenly, there is a vertical white line in the MIDI track for Addictive Drums 2. The MIDI plays then jumps to the white line.
What the heck is that? And how do I get rid of it?
Luckily, I didn't save the project once the line appeared. So, I was able to open the project without it. It has to be something simple that I clicked on but I cannot find anything about it in the Sonar documentation.

Al