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.
const width = img.width;
    const height = img.height;
    let widthScale = Math.max(
      Math.hypot(inverseTransform[0], inverseTransform[1]),
      1
    );
    let heightScale = Math.max(
      Math.hypot(inverseTransform[2], inverseTransform[3]),
      1
    );
    let paintWidth = width,
      paintHeight = height;
    let tmpCanvasId = "prescale1";
    let tmpCanvas, tmpCtx;
    while (
      (widthScale > 2 && paintWidth > 1) ||
      (heightScale > 2 && paintHeight > 1)
    ) {
      let newWidth = paintWidth,
        newHeight = paintHeight;
      if (widthScale > 2 && paintWidth > 1) {
        newWidth = Math.ceil(paintWidth / 2);
        widthScale /= paintWidth / newWidth;