visibleFragmentsForTesting static method
Exposes the subtraction directly: the cap behaviour it guarantees is a privacy property (keep the whole mask rather than under-mask), and driving it through widgets would need dozens of contrived surfaces.
Implementation
@visibleForTesting
static List<Rect> visibleFragmentsForTesting(
Rect rect, List<Rect> covered) =>
_visibleFragments(rect, covered);