Attach (recommended) or Link to PDF file here:
http://education.ohio.gov/getattachment/Topics/Special-Education/Preschool-Special-Education/ETR-Preschool-Planning-Form.pdf.aspx

- Web browser and its version: Firefox 89.0.1
- Operating system and its version: Windows 10 10.0.19043.1052
- PDF.js version: commit 5d251a3a3e7bfaa8fbfa07f0b7f9901eb3f999d9
- Is a browser extension: No

Steps to reproduce the problem:
1. Enable XFA support
2. Open form

What is the expected behavior? (add screenshot)
Shapes stay inside table cells
![image](https://user-images.githubusercontent.com/7104454/122572782-1252a580-d01c-11eb-9d0f-5fed46d6f2fe.png)

What went wrong? (add screenshot)
Shapes fall outside cells (among other issues)
![image](https://user-images.githubusercontent.com/7104454/122572885-2d251a00-d01c-11eb-8863-fa3b9969f0ea.png)

const bbox = computeBbox(this, html, availableSpace);
    const ui = this.ui ? this.ui[$toHTML]().html : null;
    if (!ui) {
      Object.assign(style, borderStyle);
      return HTMLResult.success(createWrapper(this, html), bbox);
    }
    if (this[$tabIndex]) {
      if (ui.children && ui.children[0]) {
        ui.children[0].attributes.tabindex = this[$tabIndex];
      } else {
        ui.attributes.tabindex = this[$tabIndex];
      }
    }
    if (!ui.attributes.style) {
      ui.attributes.style = Object.create(null);
    }
    if (this.ui.button) {
      Object.assign(ui.attributes.style, borderStyle);
    } else {
      Object.assign(style, borderStyle);
    }
    children.push(ui);
    if (this.value) {
      if (this.ui.imageEdit) {
        ui.children.push(this.value[$toHTML]().html);
      } else if (!this.ui.button) {
        let value = "";
        if (this.value.exData) {
          value = this.value.exData[$text]();
        } else {