Why do city start tiles vanish?
It seems like when I start a city, sometimes other city-capable tiles that are several squares away lose that capability. At first I thought the icons were just disappearing- when I play for a long time, similar graphical issues arise, like the icons at the top of the ledger and the lines between connected research topics. But I just noticed that if I take a pioneer to those areas, their ability to settle is gone.
I assume this has something to do with distance between cities, but what's the actual rule at work here? What kind of measurement do I have to go by to make sure I don't step on my own feet anymore in this regard?