http://jsfiddle.net/0bpkrnd3/4/ demonstrates that there is a very fine white line between the hover halo of a Highcharts pie chart and the pie segment. The halo color is as the same as the hover color and its stroke width is 0.
pie: {
shadow: false,
borderWidth: 0,
states: {
hover: {
color: '#ff7f00',
brightness: 0,
lineWidth: 0,
halo: {
size: 9,
opacity: 1,
attributes: {
fill: '#ff7f00',
'stroke-width': 0
}
}
}
}
}