if(clip.GetCount()) {
  printf("DC: Y1 = %i\n", y);
  const ClippingLine& s = clip.Top()[y];
  printf("DC: Y2 = %p\n", &s);
  if(s.IsEmpty()) {
    goto empty;
  }
  printf("DC: Y3 = %i\n", y);
  if(!s.IsFull()) {
    mf.Set(rg, s);
    rf = &mf;
  }
}

Results:
DC: Y1 = 525
DC: Y2 = 0x7f82b80571e8
DC: Y1 = 526
DC: Y2 = 0x7f82b80571f0
DC: Y3 = 526
DC: Y1 = 525
DC: Y2 = 0x7f82b80571e8
DC: Y1 = 526
DC: Y2 = 0x7f82b80571f0
DC: Y3 = 526
DC: MouseMove: x 16620 : y 16906 : 00000000
DC: Y1 = 526
DC: Y2 = 0x1070