[Bug] area chart's label `smartInvert` not right when position is 'inside-middle'

Version

lastest

Link to Minimal Reproduction

none

Steps to Reproduce

const spec = {
  type: 'area',
  data: {
    fields: {
      country: {
        domain: ['China', 'USA', 'EU', 'Africa'],
        sortIndex: 0
      }
    },
    values: [
      { type: 'Nail polish', country: 'Africa', value: 4229 },
      { type: 'Nail polish', country: 'EU', value: 4376 },
      { type: 'Nail polish', country: 'China', value: 3054 },
      { type: 'Nail polish', country: 'USA', value: 12814 },
      { type: 'Eyebrow pencil', country: 'Africa', value: 3932 },
      { type: 'Eyebrow pencil', country: 'EU', value: 3987 },
      { type: 'Eyebrow pencil', country: 'China', value: 5067 },
      { type: 'Eyebrow pencil', country: 'USA', value: 13012 },
      { type: 'Rouge', country: 'Africa', value: 5221 },
      { type: 'Rouge', country: 'EU', value: 3574 },
      { type: 'Rouge', country: 'China', value: 7004 },
      { type: 'Rouge', country: 'USA', value: 11624 },
      { type: 'Lipstick', country: 'Africa', value: 9256 },
      { type: 'Lipstick', country: 'EU', value: 4376 },
      { type: 'Lipstick', country: 'China', value: 9054 },
      { type: 'Lipstick', country: 'USA', value: 8814 },
      { type: 'Eyeshadows', country: 'Africa', value: 3308 },
      { type: 'Eyeshadows', country: 'EU', value: 4572 },
      { type: 'Eyeshadows', country: 'China', value: 12043 },
      { type: 'Eyeshadows', country: 'USA', value: 12998 },
      { type: 'Eyeliner', country: 'Africa', value: 5432 },
      { type: 'Eyeliner', country: 'EU', value: 3417 },
      { type: 'Eyeliner', country: 'China', value: 15067 },
      { type: 'Eyeliner', country: 'USA', value: 12321 },
      { type: 'Foundation', country: 'Africa', value: 13701 },
      { type: 'Foundation', country: 'EU', value: 5231 },
      { type: 'Foundation', country: 'China', value: 10119 },
      { type: 'Foundation', country: 'USA', value: 10342 },
      { type: 'Lip gloss', country: 'Africa', value: 4008 },
      { type: 'Lip gloss', country: 'EU', value: 4572 },
      { type: 'Lip gloss', country: 'China', value: 12043 },
      { type: 'Lip gloss', country: 'USA', value: 22998 },
      { type: 'Mascara', country: 'Africa', value: 18712 },
      { type: 'Mascara', country: 'EU', value: 6134 },
      { type: 'Mascara', country: 'China', value: 10419 },
      { type: 'Mascara', country: 'USA', value: 11261 }
    ]
  },
  title: {
    visible: true,
    text: 'Stacked area chart of cosmetic products sales'
  },
  stack: true,
  xField: 'type',
  yField: 'value',
  seriesField: 'country',
  legends: [{ visible: true, position: 'middle', orient: 'bottom' }],
  tooltip: {
    dimension: {
      updateContent: data => {
        let sum = 0;
        data.forEach(datum => {
          sum += +datum.value;
        });
        data.push({
          hasShape: 'false',
          key: 'Total',
          value: sum
        });
        return data;
      }
    }
  },
  label: {
    visible: true,
    position: 'inside-middle',
    // position: 'bottom',
    smartInvert: true
  }
};

const vchart = new VChart(spec, { dom: CONTAINER_ID });
vchart.renderSync();

// Just for the convenience of console debugging, DO NOT COPY!
window['vchart'] = vchart;

Current Behavior

Expected Behavior

Environment

- OS:
- Browser:
- Framework:

Any additional comments?

No response