I am not sure if I'm experiencing what you describe. I just see a radiating circle of black lines, no matter where I click. I decided to click a local, notable "long line" viewpoint -- Lick Observatory outside San Jose. From here, on a clear day, you can see Half Dome in Yosemite, 120mi away. I still just see a black circle.

Are you standing on a raised platform when you see the Half Dome?

This is what I get when I set the observer height to 20m, and increase the "max distance" to 300km (200km = ~124 miles so may not be enough).

https://img.incoherency.co.uk/6478

It's also possible that the half dome is too short and the sampling rate of the line-of-sight jumps over it!

It seems that sometimes it fails to load the height map, try reloading the page. You should see terrain shading if it's loaded properly.

I think the lines indicate areas that you can't see?