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):

constructor(attributes) {
    super(TEMPLATE_NS_ID, "submit",  true);
    this.embedPDF = getInteger({
      data: attributes.embedPDF,
      defaultValue: 0,
      validate: x => x === 1,
    });
    this.format = getStringOption(attributes.format, [
      "xdp",
      "formdata",
      "pdf",
      "urlencoded",
      "xfd",
      "xml",
    ]);
    this.id = attributes.id || "";
    this.target = attributes.target || "";
    this.textEncoding = getKeyword({
      data: attributes.textEncoding
        ? attributes.textEncoding.toLowerCase()
        : "",
      defaultValue: "",
      validate: k =>
        [
          "utf-8",
          "big-five",
          "fontspecific",
          "gbk",
          "gb-18030",
          "gb-2312",