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.
if (!this.contentVisible) {
      return;
    }
    const width = imgData.width;
    const height = imgData.height;
    const ctx = this.ctx;
    this.save();
    
    ctx.scale(1 / width, -1 / height);
    let imgToPaint;
    
    if (
      (typeof HTMLElement === "function" && imgData instanceof HTMLElement) ||
      !imgData.data
    ) {
      imgToPaint = imgData;
    } else {
      const tmpCanvas = this.cachedCanvases.getCanvas(
        "inlineImage",
        width,
        height
      );
      const tmpCtx = tmpCanvas.context;
      putBinaryImageData(tmpCtx, imgData, this.current.transferMaps);
      imgToPaint = tmpCanvas.canvas;
    }