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.
this.current.leading = -leading;
  }
  setFont(fontRefName, size) {
    const fontObj = this.commonObjs.get(fontRefName);
    const current = this.current;
    if (!fontObj) {
      throw new Error(`Can't find font for ${fontRefName}`);
    }
    current.fontMatrix = fontObj.fontMatrix || FONT_IDENTITY_MATRIX;
    
    
    if (current.fontMatrix[0] === 0 || current.fontMatrix[3] === 0) {
      warn("Invalid font matrix for font " + fontRefName);
    }
    
    
    if (size < 0) {
      size = -size;
      current.fontDirection = -1;
    } else {
      current.fontDirection = 1;
    }
    this.current.font = fontObj;
    this.current.fontSize = size;
    if (fontObj.isType3Font) {
      return; 
    }
    const name = fontObj.loadedName || "sans-serif";
    let bold = "normal";
    if (fontObj.black) {