Attach (recommended) or Link to PDF file here:

Configuration:
- Web browser and its version: Firefox 88.0.1
- Operating system and its version: Windows 10.0.19043.985]
- PDF.js version: (commit 706c27b9d4b3b9ff6fc3de82bcf71a0610604a1c) (also tested pr/13473 at 296bbdc4)
- Is a browser extension: N/A

Steps to reproduce the problem:
1. Enable experimental XFA
2. Open PDF
[dir2020-02_form.pdf](https://github.com/mozilla/pdf.js/files/6571084/dir2020-02_form.pdf)

What is the expected behavior? (add screenshot)
Margins of `subforms` (e.g. *General_Election_Information*, *General_Information_Other*) are honored.
![image](https://user-images.githubusercontent.com/7104454/120212605-32066300-c200-11eb-9c66-f2d05c9451ec.png)
What went wrong? (add screenshot)
Margins are not honored. 
![image](https://user-images.githubusercontent.com/7104454/120212535-1ac77580-c200-11eb-8332-9924602ef991.png)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension):

default:
          borderStyle = firstEdge.style;
          break;
      }
      style.outline = `${firstEdge.width} ${firstEdge.color} ${borderStyle}`;
      const offset = edges[0].thickness + margins[0];
      style.outlineOffset = `-${measureToString(offset)}`;
      if (isForUi) {
        style.padding = `${measureToString(offset + 1)}`;
      }
    } else {
      switch (this.presence) {
        case "invisible":
        case "hidden":
          style.borderStyle = "";
          break;
        case "inactive":
          style.borderStyle = "none";
          break;
        default:
          style.borderStyle = edgeStyles.map(s => s.style).join(" ");
          break;
      }
      style.borderWidth = edgeStyles.map(s => s.width).join(" ");
      style.borderColor = edgeStyles.map(s => s.color).join(" ");
    }
    return style;
  }
}
class Break extends XFAObject {