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):

}
        
        let mustBreak = false;
        if (this[$extra].breakingNode) {
          const node = this[$extra].breakingNode;
          this[$extra].breakingNode = null;
          if (node.targetType === "auto") {
            
            i--;
            continue;
          }
          const startNew = node.startNew === 1;
          if (node.leader) {
            leader = this[$searchNode](node.leader, node[$getParent]());
            leader = leader ? leader[0] : null;
          }
          if (node.trailer) {
            trailer = this[$searchNode](node.trailer, node[$getParent]());
            trailer = trailer ? trailer[0] : null;
          }
          let target = null;
          if (node.target) {
            target = this[$searchNode](node.target, node[$getParent]());
            target = target ? target[0] : target;
          }
          if (node.targetType === "pageArea") {
            if (startNew) {
              mustBreak = true;
            } else if (target === pageArea || !(target instanceof PageArea)) {