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.
constructor(
    canvasCtx,
    commonObjs,
    objs,
    canvasFactory,
    imageLayer,
    optionalContentConfig,
    annotationCanvasMap
  ) {
    this.ctx = canvasCtx;
    this.current = new CanvasExtraState(
      this.ctx.canvas.width,
      this.ctx.canvas.height
    );
    this.stateStack = [];
    this.pendingClip = null;
    this.pendingEOFill = false;
    this.res = null;
    this.xobjs = null;
    this.commonObjs = commonObjs;
    this.objs = objs;