drift property
What this run recorded that the run before it did not: compared,
nameMatched, the baseline it was compared against, and a count plus a
capped list of steps under changed, added and removed, each saying
in what which facets moved.
A suite that is green every pass and draws different pixels every pass says so here and nowhere else — and so does one whose status bar stopped being tinted, which no screenshot of the app's own surface can show. Null when there was no run before this one to compare against, which is not the same answer as a suite that did not move.
The lists are capped; file names the drift.json beside the report
that holds all of them. See compareScenarioRuns for how two runs are
paired, which is the part worth knowing before trusting a count.
Not part of what the harness writes to run.json: a report is about the
run it belongs to, and which run came before it is not a fact about that
run. It has its own file — scenarioRunDriftFile.
Implementation
final ScenarioRunDrift? drift;