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):

}
  if (parent.h && node.h) {
    node.h = Math.min(parent.h, node.h);
  }
  if (parent.layout && parent.layout !== "position") {
    
    node.x = node.y = 0;
    if (parent.layout === "tb") {
      if (
        parent.w !== "" &&
        (node.w === "" || node.w === 0 || node.w > parent.w)
      ) {
        node.w = parent.w;
      }
    }
  }
  if (node.layout === "position") {
    
    
    node.minW = node.minH = 0;
    node.maxW = node.maxH = Infinity;
  } else {
    if (node.layout === "table") {
      if (node.w === "" && Array.isArray(node.columnWidths)) {
        node.w = node.columnWidths.reduce((a, x) => a + x, 0);
      }
    }
  }
}
function layoutClass(node) {