Auto merge of #28880 - wusyong:fix-footer, r=delan

Fix absolute descendents being replaced when retreiving root

<!-- Please describe your changes on the following line: -->
The cause of #16410 is because calling `try_get_layout_root` will replace its `abs_descendants` even itself isn't absolute positioned. So I pushed them instead if the root isn't absolute. Also add a few docs and comments to help understand how block size of absolute flow is being calculated.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #16410 (GitHub issue number if applicable)

<!-- Either: -->
- [x] There are tests for these changes:
Reduced testcase from the issue.
```html
<style>
  html {
    min-height: 100%;
    position: relative;
  }
  div {
    position: absolute;
    bottom: 0;
    width: 100%;
    height: 25px;
    border: 1px solid black;
  }
</style>
<div>
</div>
```

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