How do I keep the scroll bar from disappearing in Mojave

I just upgraded to Mojave. I'm on a MacBook Pro

The scroll bar keeps disappearing. I have to change the size of the window I am in to make it

reappear, but sometimes by the time I get my curser to it it disappears again.

A time suck, and and annoying. Any help. I'm sure it's something basic.

Thanks everyone.