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.
const charSpacing = current.charSpacing;
    const wordSpacing = current.wordSpacing;
    const fontDirection = current.fontDirection;
    const textHScale = current.textHScale * fontDirection;
    const glyphsLength = glyphs.length;
    const vertical = font.vertical;
    const spacingDir = vertical ? 1 : -1;
    const defaultVMetrics = font.defaultVMetrics;
    const widthAdvanceScale = fontSize * current.fontMatrix[0];
    const simpleFillText =
      current.textRenderingMode === TextRenderingMode.FILL &&
      !font.disableFontFace &&
      !current.patternFill;
    ctx.save();
    ctx.transform.apply(ctx, current.textMatrix);
    ctx.translate(current.x, current.y + current.textRise);
    if (fontDirection > 0) {
      ctx.scale(textHScale, -1);
    } else {
      ctx.scale(textHScale, 1);
    }
    let patternTransform;
    if (current.patternFill) {
      ctx.save();
      const pattern = current.fillColor.getPattern(
        ctx,
        this,
        ctx.mozCurrentTransformInverse,
        PathType.FILL
      );