sclass ImageSurface_HighlightRegion > ImageSurfaceMouseHandler is G2Drawable {
  settable RegionPainter regionPainter = new RegionBoundingBoxPainter;
  FastRegions_BWImage regions;
  int iHighlightedRegion;
  Pt lastPt;
  
  *(ImageSurface is) {
    if (containsInstance(is.tools, getClass())) ret;
    register(is);
    is.overlays.add(this);
  }
  
  public void close :: after {
    is.overlays.remove(this);
  }

  public void mouseMoved aka mouseEntered(MouseEvent e) {
    lastPt = getPt(e);
    update();
  }
  
  public void mouseExited(MouseEvent e) {
    lastPt = null;
    update();
  }
  
  int newRegion() {
    if (lastPt == null || regions == null) ret 0;
    ret regions.regionAt(lastPt.x, lastPt.y);
  }
  
  void update {
    int r = newRegion();
    if (iHighlightedRegion != r) {
      iHighlightedRegion = r;
      is.repaint();
    }
  }
  
  void setRegions(FastRegions_BWImage regions) {
    this.regions = regions;
    iHighlightedRegion = -abs(iHighlightedRegion);
    update();
  }
  
  public void drawOn(Graphics2D g) {
    if (iHighlightedRegion == 0 || regionPainter == null) ret;
    regionPainter.zoomX(is.getZoomX());
    regionPainter.zoomY(is.getZoomY());
    regionPainter.g(g); afterwards { regionPainter.g(null); }
    regionPainter.regions(regions).iRegion(iHighlightedRegion);
    regionPainter.drawRegion();
  }
}

Began life as a copy of #1033810