I have reported this one before but I see it still happens in the beta so thought I would mention it.
When using Safari and in Cornerstone, the breakpoints activate at wrong screen sizes. This doesn’t happen with Chrome or Brave. I have tried in the past creating a clean macOS user account to make sure it wasn’t to do with a browser plugin.
( It happens on my own staging site as well as on https://beta.theme.co )
So the breakpoint should happen at the transition from 979 to 980 px but actually happens at 993 / 994 px
See screen recording video and look at when the screen size icons on the bottom row change to blue: