Program: 67651
Project: 67651
Buggy source:
public void activate(){
  if (isActive())   return;
  super.activate();
  refreshPageClip();
  getFigure().addMouseMotionListener(new MouseMotionListener.Stub(){
    public void mouseEntered(    MouseEvent me){
      addGuideFeedBack();
    }
    public void mouseExited(    MouseEvent me){
      delayRemoveGuideFeedBack();
    }
    public void mouseHover(    MouseEvent me){
      addGuideFeedBack();
    }
    public void mouseMoved(    MouseEvent me){
      addGuideFeedBack();
    }
  }
);
  getFigure().setFocusTraversable(true);
  updateLayoutPreference();
  Display.getCurrent().asyncExec(new Runnable(){
    public void run(){
      if (!(getModel() instanceof DesignElementHandle)) {
        return;
      }
      if (isDelete()) {
        return;
      }
      EditPart parent=getParent();
      while (parent != null) {
        if (parent instanceof RootEditPart) {
          break;
        }
        parent=parent.getParent();
      }
      if (parent == null) {
        return;
      }
      if (((DeferredGraphicalViewer)getViewer()).getFigureCanvas() == null) {
        return;
      }
      DesignElementHandle handle=(DesignElementHandle)getModel();
      EditPart part=ReportElementEditPart.this;
      while (part != null && !(part instanceof RootEditPart)) {
        part=part.getParent();
      }
      if (getModelAdapter() == null || part == null) {
        return;
      }
      Object[] backGroundPosition=getBackgroundPosition(handle);
      Object xPosition=backGroundPosition[0];
      Object yPosition=backGroundPosition[1];
      boolean needRefresh=false;
      if (xPosition instanceof DimensionValue) {
        needRefresh=true;
      }
      if (yPosition instanceof DimensionValue) {
        needRefresh=true;
      }
      if (needRefresh) {
        refreshVisuals();
      }
    }
  }
);
}

Fixed version (ground truth):
public void activate(){
  if (isActive())   return;
  super.activate();
  refreshPageClip();
  getFigure().addMouseMotionListener(new MouseMotionListener.Stub(){
    public void mouseEntered(    MouseEvent me){
      addGuideFeedBack();
    }
    public void mouseExited(    MouseEvent me){
      delayRemoveGuideFeedBack();
    }
    public void mouseHover(    MouseEvent me){
      addGuideFeedBack();
    }
    public void mouseMoved(    MouseEvent me){
      addGuideFeedBack();
    }
  }
);
  getFigure().setFocusTraversable(true);
  updateLayoutPreference();
  Display.getCurrent().asyncExec(new Runnable(){
    public void run(){
      if (!(getModel() instanceof DesignElementHandle)) {
        return;
      }
      if (isDelete()) {
        return;
      }
      EditPart parent=getParent();
      while (parent != null) {
        if (parent instanceof RootEditPart) {
          break;
        }
        parent=parent.getParent();
      }
      if (parent == null) {
        return;
      }
      if (((DeferredGraphicalViewer)getViewer()).getFigureCanvas() == null) {
        return;
      }
      DesignElementHandle handle=(DesignElementHandle)getModel();
      EditPart part=ReportElementEditPart.this;
      while (part != null && !(part instanceof RootEditPart)) {
        part=part.getParent();
      }
      if (getModelAdapter() == null || part == null) {
        return;
      }
      Object[] backGroundPosition=getBackgroundPosition(handle);
      Object xPosition=backGroundPosition[0];
      Object yPosition=backGroundPosition[1];
      boolean needRefresh=false;
      if (xPosition instanceof DimensionValue) {
        needRefresh=true;
      }
      if (yPosition instanceof DimensionValue) {
        needRefresh=true;
      }
      if (isPercentageValue(handle.getProperty(StyleHandle.MARGIN_LEFT_PROP))) {
        needRefresh=true;
      }
      if (isPercentageValue(handle.getProperty(StyleHandle.MARGIN_RIGHT_PROP))) {
        needRefresh=true;
      }
      if (needRefresh) {
        refreshVisuals();
      }
    }
  }
);
}