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.
if (!group.isolated) {
      info("TODO: Support non-isolated groups.");
    }
    
    
    if (group.knockout) {
      warn("Knockout groups not supported.");
    }
    const currentTransform = currentCtx.mozCurrentTransform;
    if (group.matrix) {
      currentCtx.transform.apply(currentCtx, group.matrix);
    }
    if (!group.bbox) {
      throw new Error("Bounding box is required.");
    }
    
    
    let bounds = Util.getAxialAlignedBoundingBox(
      group.bbox,
      currentCtx.mozCurrentTransform
    );