If the yellow part of the animation below is what you saw…

…then the way I figured it out was:
- I think Scott Kelby uses a Mac.
- Johan’s reply mentioned a blue line on his Mac, and that got me thinking because…
- Blue is the default Accent Color on macOS.
Putting it all together, the demo above shows the Accent Color being changed to Yellow in the General pane of macOS System Preferences, which affects all Mac applications. The Highlight Color also seems to affect Lightroom Classic list dragging in the same way. If you’re using Windows, I have to imagine Windows offers similar options.
If that is not what you saw, I’m not sure what else it could be.