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):

case 1:
          return {
            width: availableSpace.width,
            height: availableSpace.height - node[$extra].height,
          };
        default:
          return {
            width: Infinity,
            height: availableSpace.height - node[$extra].prevHeight,
          };
      }
    case "rl-row":
    case "row":
      const width = node[$extra].columnWidths
        .slice(node[$extra].currentColumn)
        .reduce((a, x) => a + x);
      return { width, height: availableSpace.height };
    case "table":
    case "tb":
      return {
        width: availableSpace.width,
        height: availableSpace.height - node[$extra].height,
      };
    case "position":
    default:
      return availableSpace;
  }
}
export { addHTML, flushHTML, getAvailableSpace };