This is a regression from #12864. The underscore is now too thin when the document is zoomed in.

URL: https://github.com/mozilla/pdf.js/files/2321540/issue6127.pdf
PDF.js 2.7.526: https://i.imgur.com/T4mIpct.png
PDF.js 2.7.510: https://i.imgur.com/nbwEpGT.png
Adobe Acrobat Reader DC 2020.013.20074: https://i.imgur.com/wDFBf3R.png

The screenshots were taken at 400% zoom.
patternTransform = ctx.mozCurrentTransform;
      ctx.restore();
      ctx.fillStyle = pattern;
    }
    let lineWidth = current.lineWidth;
    let resetLineWidthToOne = false;
    const scale = current.textMatrixScale;
    if (scale === 0 || lineWidth === 0) {
      const fillStrokeMode =
        current.textRenderingMode & TextRenderingMode.FILL_STROKE_MASK;
      if (
        fillStrokeMode === TextRenderingMode.STROKE ||
        fillStrokeMode === TextRenderingMode.FILL_STROKE
      ) {
        this._cachedGetSinglePixelWidth = null;
        lineWidth = this.getSinglePixelWidth();
        resetLineWidthToOne = lineWidth < 0;
      }
    } else {
      lineWidth /= scale;
    }
    if (fontSizeScale !== 1.0) {
      ctx.scale(fontSizeScale, fontSizeScale);
      lineWidth /= fontSizeScale;
    }
    ctx.lineWidth = lineWidth;
    let x = 0,
      i;
    for (i = 0; i < glyphsLength; ++i) {
      const glyph = glyphs[i];