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.
const canvasBounds = [
      0,
      0,
      currentCtx.canvas.width,
      currentCtx.canvas.height,
    ];
    bounds = Util.intersect(bounds, canvasBounds) || [0, 0, 0, 0];
    
    
    const offsetX = Math.floor(bounds[0]);
    const offsetY = Math.floor(bounds[1]);
    let drawnWidth = Math.max(Math.ceil(bounds[2]) - offsetX, 1);
    let drawnHeight = Math.max(Math.ceil(bounds[3]) - offsetY, 1);
    let scaleX = 1,
      scaleY = 1;
    if (drawnWidth > MAX_GROUP_SIZE) {
      scaleX = drawnWidth / MAX_GROUP_SIZE;
      drawnWidth = MAX_GROUP_SIZE;
    }
    if (drawnHeight > MAX_GROUP_SIZE) {
      scaleY = drawnHeight / MAX_GROUP_SIZE;
      drawnHeight = MAX_GROUP_SIZE;
    }
    this.current.startNewPathAndClipBox([0, 0, drawnWidth, drawnHeight]);
    let cacheId = "groupAt" + this.groupLevel;
    if (group.smask) {
      
      cacheId += "_smask_" + (this.smaskCounter++ % 2);
    }
    const scratchCanvas = this.cachedCanvases.getCanvas(