Pseudocode for ticking the refresh driver as quickly as possible when a key event comes in

early-tick.cpp

bool mPendingTick = false;
bool mAlreadyTickedThisVsyncInterval = false;

void ScheduleTick() {
  if (mPendingTick) {
    return;
  }

  mPendingTick = true;
  EnsureListeningToVsync([](){ OnVSync(); });
  if (!mAlreadyTickedThisVsyncInterval) {
    ScheduleImmediatelyWithVsyncPriority([](){ Tick(); });
  }
}

void OnVSync() {
  // This notification indicates the start of a new “vsync interval”.
  mAlreadyTickedThisVsyncInterval = false;

  if (mPendingTick) {
    Tick();
  }
}

void Tick() {
  MOZ_RELEASE_ASSERT(mPendingTick);
  MOZ_RELEASE_ASSERT(!mAlreadyTickedThisVsyncInterval);

  mAlreadyTickedThisVsyncInterval = true;
  mPendingTick = false;
  DoTheWork();
}