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.
accent.fontChar,
              scaledAccentX,
              scaledAccentY,
              patternTransform,
              resetLineWidthToOne
            );
          }
        }
      }
      let charWidth;
      if (vertical) {
        charWidth = width * widthAdvanceScale - spacing * fontDirection;
      } else {
        charWidth = width * widthAdvanceScale + spacing * fontDirection;
      }
      x += charWidth;
      if (restoreNeeded) {
        ctx.restore();
      }
    }
    if (vertical) {
      current.y -= x;
    } else {
      current.x += x * textHScale;
    }
    ctx.restore();
    this.compose();
    return undefined;
  }
  showType3Text(glyphs) {