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.
constructor(canvasFactory) {
    this.canvasFactory = canvasFactory;
    this.cache = Object.create(null);
  }
  getCanvas(id, width, height, trackTransform) {
    let canvasEntry;
    if (this.cache[id] !== undefined) {
      canvasEntry = this.cache[id];
      this.canvasFactory.reset(canvasEntry, width, height);
      
      canvasEntry.context.setTransform(1, 0, 0, 1, 0, 0);
    } else {
      canvasEntry = this.canvasFactory.create(width, height);
      this.cache[id] = canvasEntry;
    }
    if (trackTransform) {
      addContextCurrentTransform(canvasEntry.context);
    }
    return canvasEntry;
  }
  clear() {