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.
transform,
    viewport,
    transparency = false,
    background = null,
  }) {
    
    
    
    
    
    const width = this.ctx.canvas.width;
    const height = this.ctx.canvas.height;
    this.ctx.save();
    this.ctx.fillStyle = background || "rgb(255, 255, 255)";
    this.ctx.fillRect(0, 0, width, height);
    this.ctx.restore();
    if (transparency) {
      const transparentCanvas = this.cachedCanvases.getCanvas(
        "transparent",
        width,