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):

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          width = 0;
        } else {
          width = node.minW;
        }
      } else {
        width = Math.min(node.maxW, availableSpace.width);
      }
      html.attributes.style.width = measureToString(width);
    }
    let height = node.h;
    if (height === "") {
      if (node.maxH === 0) {
        const parent = node[$getParent]();
        if (parent.layout === "position" && parent.h !== "") {
          height = 0;
        } else {
          height = node.minH;
        }