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.
maskCtx,
            this,
            ctx.mozCurrentTransformInverse,
            PathType.FILL
          )
        : fillColor;
      maskCtx.fillRect(0, 0, width, height);
      maskCtx.restore();
      ctx.save();
      ctx.transform.apply(ctx, image.transform);
      ctx.scale(1, -1);
      ctx.drawImage(maskCanvas.canvas, 0, 0, width, height, 0, -1, 1, 1);
      ctx.restore();
    }
    this.compose();
  }
  paintImageXObject(objId) {
    if (!this.contentVisible) {
      return;
    }
    const imgData = objId.startsWith("g_")
      ? this.commonObjs.get(objId)
      : this.objs.get(objId);
    if (!imgData) {
      warn("Dependent image isn't ready yet");
      return;
    }
    this.paintInlineImageXObject(imgData);
  }
  paintImageXObjectRepeat(objId, scaleX, scaleY, positions) {