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.save();
    if (Array.isArray(rect) && rect.length === 4) {
      const width = rect[2] - rect[0];
      const height = rect[3] - rect[1];
      if (hasOwnCanvas && this.annotationCanvasMap) {
        transform = transform.slice();
        transform[4] -= rect[0];
        transform[5] -= rect[1];
        rect = rect.slice();
        rect[0] = rect[1] = 0;
        rect[2] = width;
        rect[3] = height;
        const [scaleX, scaleY] = Util.singularValueDecompose2dScale(
          this.ctx.mozCurrentTransform
        );
        const { viewportScale } = this;
        const canvasWidth = Math.ceil(
          width * this.outputScaleX * viewportScale
        );
        const canvasHeight = Math.ceil(
          height * this.outputScaleY * viewportScale
        );
        this.annotationCanvas = this.canvasFactory.create(
          canvasWidth,
          canvasHeight
        );
        const { canvas, context } = this.annotationCanvas;
        canvas.style.width = `calc(${width}px * var(--viewport-scale-factor))`;