A GPS fix is a rumour. It arrives every second or so with an error radius, and if you join them all you get a line that shivers at traffic lights and cuts corners through buildings. The Route Shoals on Cadence's island are where I learned to distrust the raw stream.
The run screen takes the fixes, drops the ones with poor accuracy or an impossible jump from the last, and only then extends the line. Distance is the sum of the accepted legs, pace is a short moving average so the number does not flicker, and a kilometre marker is dropped when the total crosses a whole number, interpolated between the two fixes that straddle it.
The map is dark on purpose, so the route is the brightest thing on it, and the numbers sit on one bottom sheet that suits a thumb. The least glamorous decision was to keep recording when the screen locks and to save the run as it goes. A run is the one thing the app must never lose.