Program: 97810
Project: 97810
Buggy source:
private void fillDown(SnipeData v,Location<World> targetBlock){
  double brushSize=v.getBrushSize();
  double brushSizeSquared=brushSize * brushSize;
  int minx=GenericMath.floor(targetBlock.getBlockX() - brushSize);
  int maxx=GenericMath.floor(targetBlock.getBlockX() + brushSize) + 1;
  int minz=GenericMath.floor(targetBlock.getBlockZ() - brushSize);
  int maxz=GenericMath.floor(targetBlock.getBlockZ() + brushSize) + 1;
  this.undo=new Undo(GenericMath.floor(4 * Math.PI * (brushSize + 1)* (brushSize + 1)* (brushSize + 1) / 3));
  for (int x=minx; x <= maxx; x++) {
    double xs=(minx - x) * (minx - x);
    for (int z=minz; z <= maxz; z++) {
      double zs=(minz - z) * (minz - z);
      if (xs + zs < brushSizeSquared) {
        int y=targetBlock.getBlockY();
        if (this.fromExisting) {
          for (int y0=-v.getVoxelHeight(); y0 < v.getVoxelHeight(); y0++) {
            if (this.world.getBlock(x,y + y0,z) != v.getReplaceIdState()) {
              y+=y0 - 1;
              break;
            }
          }
        }
        for (; y >= 0; y--) {
          if (this.replace != PerformerType.NONE) {
            if (!perform(v,x,y,z)) {
              break;
            }
          }
 else {
            BlockState current=this.world.getBlock(x,y,z);
            if (current.getType() == BlockTypes.AIR) {
              perform(v,x,y,z);
            }
 else             if (this.fillLiquid) {
              Optional<MatterProperty> matter=current.getProperty(MatterProperty.class);
              if (matter.isPresent()) {
                Matter m=matter.get().getValue();
                if (m == Matter.LIQUID) {
                  perform(v,x,y,z);
                }
              }
            }
          }
        }
      }
    }
  }
}

Fixed version (ground truth):
private void fillDown(SnipeData v,Location<World> targetBlock){
  double brushSize=v.getBrushSize();
  double brushSizeSquared=brushSize * brushSize;
  int tx=targetBlock.getBlockX();
  int tz=targetBlock.getBlockZ();
  int minx=GenericMath.floor(targetBlock.getBlockX() - brushSize);
  int maxx=GenericMath.floor(targetBlock.getBlockX() + brushSize) + 1;
  int minz=GenericMath.floor(targetBlock.getBlockZ() - brushSize);
  int maxz=GenericMath.floor(targetBlock.getBlockZ() + brushSize) + 1;
  this.undo=new Undo(GenericMath.floor(4 * Math.PI * (brushSize + 1)* (brushSize + 1)* (brushSize + 1) / 3));
  for (int x=minx; x <= maxx; x++) {
    double xs=(tx - x) * (tx - x);
    for (int z=minz; z <= maxz; z++) {
      double zs=(tz - z) * (tz - z);
      if (xs + zs < brushSizeSquared) {
        int y=targetBlock.getBlockY();
        if (this.fromExisting) {
          for (int y0=-v.getVoxelHeight(); y0 < v.getVoxelHeight(); y0++) {
            if (this.world.getBlock(x,y + y0,z) != v.getReplaceIdState()) {
              y+=y0 - 1;
              break;
            }
          }
        }
        for (; y >= 0; y--) {
          if (this.replace != PerformerType.NONE) {
            if (!perform(v,x,y,z)) {
              break;
            }
          }
 else {
            BlockState current=this.world.getBlock(x,y,z);
            if (current.getType() == BlockTypes.AIR) {
              perform(v,x,y,z);
            }
 else             if (this.fillLiquid) {
              Optional<MatterProperty> matter=current.getProperty(MatterProperty.class);
              if (matter.isPresent()) {
                Matter m=matter.get().getValue();
                if (m == Matter.LIQUID) {
                  perform(v,x,y,z);
                }
              }
            }
          }
        }
      }
    }
  }
}