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):

"qrcode",
        "rfid",
        "rss14",
        "rss14expanded",
        "rss14limited",
        "rss14stacked",
        "rss14stackedomni",
        "rss14truncated",
        "telepen",
        "ucc128",
        "ucc128random",
        "ucc128sscc",
        "upca",
        "upcaadd2",
        "upcaadd5",
        "upcapwcd",
        "upce",
        "upceadd2",
        "upceadd5",
        "upcean2",
        "upcean5",
        "upsmaxicode",
      ]
    );
    this.upsMode = getStringOption(attributes.upsMode, [
      "usCarrier",
      "internationalCarrier",
      "secureSymbol",
      "standardSymbol",
    ]);