vsyncStartUs property
The vsync signal timestamp for this frame, from
FrameTiming.timestampInMicroseconds(FramePhase.vsyncStart).
NEEDS-LIVE-VALIDATION: the engine documents this timestamp only as
microseconds "from some epoch" shared across every FrameTiming field,
explicitly disclaiming parity with DateTime's epoch, and says nothing
about parity with atUs's source, FlutterTimeline.now (Timeline.now
on the VM, performance.now() on web). A widget test only ever
supplies FrameTiming through its unit-test factory constructor, which
takes raw integers the test itself chooses, so there is no route from
flutter test to compare a real engine-reported vsync timestamp
against a real FlutterTimeline.now read. Confirming or refuting clock
parity, on either the VM or web, needs a driven run against a live
engine rather than a unit test.
Implementation
final int? vsyncStartUs;