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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components/layout_thread/lib.rs

@@ -44,7 +44,7 @@ use layout::context::RegisteredPainter;
 use layout::context::RegisteredPainters;
 use layout::display_list::items::WebRenderImageInfo;
 use layout::display_list::{IndexableText, ToLayout};
-use layout::flow::{Flow, GetBaseFlow, ImmutableFlowUtils, MutableOwnedFlowUtils};
+use layout::flow::{Flow, FlowFlags, GetBaseFlow, ImmutableFlowUtils, MutableOwnedFlowUtils};
 use layout::flow_ref::FlowRef;
 use layout::incremental::{RelayoutMode, SpecialRestyleDamage};
 use layout::layout_debug;
@@ -950,11 +950,20 @@ impl LayoutThread {
         let mut flow = match result {
             ConstructionResult::Flow(mut flow, abs_descendants) => {
                 // Note: Assuming that the root has display 'static' (as per
-                // CSS Section 9.3.1). Otherwise, if it were absolutely
-                // positioned, it would return a reference to itself in
-                // `abs_descendants` and would lead to a circular reference.
-                // Set Root as CB for any remaining absolute descendants.
-                flow.set_absolute_descendants(abs_descendants);
+                // CSS Section 9.3.1). If it was absolutely positioned,
+                // it would return a reference to itself in `abs_descendants`
+                // and would lead to a circular reference. Otherwise, we
+                // set Root as CB and push remaining absolute descendants.
+                if flow
+                    .base()
+                    .flags
+                    .contains(FlowFlags::IS_ABSOLUTELY_POSITIONED)
+                {
+                    flow.set_absolute_descendants(abs_descendants);
+                } else {
+                    flow.push_absolute_descendants(abs_descendants);
+                }
+
                 flow
             },
             _ => return None,