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.
height,
        true
      );
      this.compositeCtx = this.ctx;
      this.transparentCanvas = transparentCanvas.canvas;
      this.ctx = transparentCanvas.context;
      this.ctx.save();
      
      
      this.ctx.transform.apply(this.ctx, this.compositeCtx.mozCurrentTransform);
    }
    this.ctx.save();
    resetCtxToDefault(this.ctx);
    if (transform) {
      this.ctx.transform.apply(this.ctx, transform);
      this.outputScaleX = transform[0];
      this.outputScaleY = transform[0];
    }
    this.ctx.transform.apply(this.ctx, viewport.transform);
    this.viewportScale = viewport.scale;
    this.baseTransform = this.ctx.mozCurrentTransform.slice();
    this._combinedScaleFactor = Math.hypot(
      this.baseTransform[0],
      this.baseTransform[2]
    );
    if (this.imageLayer) {
      this.imageLayer.beginLayout();
    }
  }
  executeOperatorList(