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):

i--;
              continue;
            } else {
              
              targetPageArea = target;
              mustBreak = true;
            }
          } else if (
            target === "contentArea" ||
            !(target instanceof ContentArea)
          ) {
            
            i--;
            continue;
          }
        }
        html = root[$flushHTML]();
        if (html) {
          htmlContentAreas[i].children.push(html);
        }
        if (mustBreak) {
          break;
        }
      }
      this[$extra].pageNumber += 1;
      pageArea = targetPageArea || pageArea[$getNextPage]();
    }
  }
}
class Text extends ContentObject {