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.
return this._getLineWidth();
      },
    });
  } catch (_) {}
  Object.defineProperty(ctx, "mozCurrentTransform", {
    get: function getCurrentTransform() {
      return this._transformMatrix;
    },
  });
  Object.defineProperty(ctx, "mozCurrentTransformInverse", {
    get: function getCurrentTransformInverse() {
      
      
      
      const [a, b, c, d, e, f] = this._transformMatrix;
      const ad_bc = a * d - b * c;
      const bc_ad = b * c - a * d;
      return [
        d / ad_bc,
        b / bc_ad,
        c / bc_ad,
        a / ad_bc,
        (d * e - c * f) / bc_ad,
        (b * e - a * f) / ad_bc,
      ];
    },
  });
  ctx.save = function ctxSave() {
    const old = this._transformMatrix;
    this._transformStack.push(old);