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 current = this.current;
    const font = current.font;
    if (font.isType3Font) {
      return this.showType3Text(glyphs);
    }
    const fontSize = current.fontSize;
    if (fontSize === 0) {
      return undefined;
    }
    const ctx = this.ctx;
    const fontSizeScale = current.fontSizeScale;