Where are the quick preview buttons?

I've run into this before as well. Unfortunately, there isn't a setting to turn the quick preview icons on or off.

What worked for me was closing the project and opening another one where the icons were displaying correctly. After that, I reopened the original project, and the icons were back. It seems to be just a temporary glitch. It's worth noting that the project where I experienced this issue was inherited from someone else. That may or may not be related, but I suspect it's a glitch in the latest Storyline build.

Worth to try:

Specifically... 

How do I turn these on and off?