public void handleDrop(Modeler modeler, Transfer transfer, EditPart editPart) {
    //I get the inputEvent in an other method
    mouseLocation = new Point(inputEvent.x, inputEvent.y);
    //The edit part is the root editPart on the editor
    IFigure fig = ((GraphicalEditPart)editPart).getFigure();
    fig.translateToRelative(mouseLocation);
    EditPart part = editPart.getViewer().findObjectAt(mouseLocation);
    //[...]
}

setMouseLocation(new Point(getInputEvent().x, getInputEvent().y));
DropTarget target = (DropTarget)getInputEvent().widget;

org.eclipse.swt.graphics.Point relativePoint = target.getControl().toControl(new org.eclipse.swt.graphics.Point(getInputEvent().x,getInputEvent().y));

getMouseLocation().setLocation(relativePoint.x, relativePoint.y);
EditPart part = editPart.getViewer().findObjectAt(getMouseLocation());