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.
let pattern;
    if (IR[0] === "TilingPattern") {
      const color = IR[1];
      const baseTransform =
        this.baseTransform || this.ctx.mozCurrentTransform.slice();
      const canvasGraphicsFactory = {
        createCanvasGraphics: ctx => {
          return new CanvasGraphics(
            ctx,
            this.commonObjs,
            this.objs,
            this.canvasFactory
          );
        },
      };
      pattern = new TilingPattern(
        IR,
        color,
        this.ctx,
        canvasGraphicsFactory,
        baseTransform
      );
    } else {
      pattern = this._getPattern(IR[1], IR[2]);
    }
    return pattern;
  }
  setStrokeColorN() {