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):

} else {
        height = Math.min(node.maxH, availableSpace.height);
      }
      html.attributes.style.height = measureToString(height);
    }
    bbox = [node.x, node.y, width, height];
  }
  return bbox;
}
function fixDimensions(node) {
  const parent = node[$getParent]();
  if (parent.layout && parent.layout.includes("row")) {
    const extra = parent[$extra];
    const colSpan = node.colSpan;
    let width;
    if (colSpan === -1) {
      width = extra.columnWidths
        .slice(extra.currentColumn)
        .reduce((a, w) => a + w, 0);
    } else {
      width = extra.columnWidths
        .slice(extra.currentColumn, extra.currentColumn + colSpan)
        .reduce((a, w) => a + w, 0);
    }
    if (!isNaN(width)) {
      node.w = width;
    }
  }
  if (parent.w && node.w) {
    node.w = Math.min(parent.w, node.w);