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.
this.canvasFactory = canvasFactory;
    this.imageLayer = imageLayer;
    this.groupStack = [];
    this.processingType3 = null;
    
    
    this.baseTransform = null;
    this.baseTransformStack = [];
    this.groupLevel = 0;
    this.smaskStack = [];
    this.smaskCounter = 0;
    this.tempSMask = null;
    this.suspendedCtx = null;
    this.contentVisible = true;
    this.markedContentStack = [];
    this.optionalContentConfig = optionalContentConfig;
    this.cachedCanvases = new CachedCanvases(this.canvasFactory);
    this.cachedPatterns = new Map();
    this.annotationCanvasMap = annotationCanvasMap;
    this.viewportScale = 1;
    this.outputScaleX = 1;
    this.outputScaleY = 1;
    if (canvasCtx) {
      
      
      addContextCurrentTransform(canvasCtx);
    }
    this._cachedGetSinglePixelWidth = null;
  }
  beginDrawing({