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.
cacheId,
      drawnWidth,
      drawnHeight,
      true
    );
    const groupCtx = scratchCanvas.context;
    
    
    groupCtx.scale(1 / scaleX, 1 / scaleY);
    groupCtx.translate(-offsetX, -offsetY);
    groupCtx.transform.apply(groupCtx, currentTransform);
    if (group.smask) {
      
      this.smaskStack.push({
        canvas: scratchCanvas.canvas,
        context: groupCtx,
        offsetX,
        offsetY,
        scaleX,
        scaleY,
        subtype: group.smask.subtype,
        backdrop: group.smask.backdrop,
        transferMap: group.smask.transferMap || null,
        startTransformInverse: null, 
      });
    } else {
      
      
      currentCtx.setTransform(1, 0, 0, 1, 0, 0);
      currentCtx.translate(offsetX, offsetY);