Function
The run in progress
The side that was tapped
The learner's step, used only if the run turns around
The run after the tap
A tap on the run's own side adds a step, counted in the run's own
stepSeconds — stepSeconds here is ignored, which is what keeps a run
in flight honest when the learner changes the setting mid-run.
A tap on the other side turns the run around: a fresh one-step run anchored where the old run was heading, so the label starts counting again, as the YouTube app's does. That run is new, so it does take the step passed in.
Adds one tap to a run.