Mysterious highlight on the row the cursor sits on

• iTerm2 version: 3.1.5
• OS version: 10.13.5com.googlecode.iterm2.plist

I'm unable to reproduce the problem, and that's the problem. On occasion I'll hit a keystroke I didn't mean to, and a highlight will show up on the row of my terminal where my cursor currently is (screenshot attached). There's probably a simple answer, but I couldn't find it in the docs. How do you disable this or turn it back off?