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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<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

components/layout/block.rs

@@ -903,7 +903,7 @@ impl BlockFlow {
     ///   10.6.7.
     ///
     /// For absolute flows, we store the calculated content block-size for the flow. We defer the
-    /// calculation of the other values until a later traversal.
+    /// calculation of the other values until a later traversal at root flow.
     ///
     /// When `fragmentation_context` is given (not `None`), this should fit as much of the content
     /// as possible within the available block size.
@@ -1200,6 +1200,8 @@ impl BlockFlow {
                 .contains(FlowFlags::IS_ABSOLUTELY_POSITIONED)
             {
                 self.propagate_early_absolute_position_info_to_children();
+                // Return early until the absolute flow tree traversal at root flow.
+                // Assigning block size for absolute flow will happen in `traverse_absolute_flows` below.
                 return None;
             }
 

components/layout/flow.rs

@@ -557,6 +557,11 @@ pub trait MutableOwnedFlowUtils {
     /// Set this flow as the Containing Block for all the absolute descendants.
     fn set_absolute_descendants(&mut self, abs_descendants: AbsoluteDescendants);
 
+    /// Push absolute descendants to this flow.
+    ///
+    /// Set this flow as the Containing Block for the provided absolute descendants.
+    fn push_absolute_descendants(&mut self, abs_descendants: AbsoluteDescendants);
+
     /// Sets the flow as the containing block for all absolute descendants that have been marked
     /// as having reached their containing block. This is needed in order to handle cases like:
     ///
@@ -1364,6 +1369,26 @@ impl MutableOwnedFlowUtils for FlowRef {
         }
     }
 
+    /// Push absolute descendants for this flow.
+    ///
+    /// Set yourself as the Containing Block for the provided absolute descendants.
+    ///
+    /// This is called when retreiving layout root if it's not absolute positioned. We can't just
+    /// call `set_absolute_descendants` because it might contain other abs_descendants already.
+    /// We push descendants instead of replace it since it won't cause circular reference.
+    fn push_absolute_descendants(&mut self, mut abs_descendants: AbsoluteDescendants) {
+        let this = self.clone();
+        let base = FlowRef::deref_mut(self).mut_base();
+
+        for descendant_link in abs_descendants.descendant_links.iter_mut() {
+            debug_assert!(!descendant_link.has_reached_containing_block);
+            let descendant_base = FlowRef::deref_mut(&mut descendant_link.flow).mut_base();
+            descendant_base.absolute_cb.set(this.clone());
+        }
+
+        base.abs_descendants.push_descendants(abs_descendants);
+    }
+
     /// Sets the flow as the containing block for all absolute descendants that have been marked
     /// as having reached their containing block. This is needed in order to handle cases like:
     ///

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.
+                // 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,

tests/wpt/metadata/css/CSS2/positioning/abspos-containing-block-005.xht.ini

@@ -1,3 +0,0 @@
-[abspos-containing-block-005.xht]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata/css/CSS2/positioning/abspos-containing-block-006.xht.ini

@@ -1,3 +0,0 @@
-[abspos-containing-block-006.xht]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata/css/filter-effects/filtered-html-is-not-container.html.ini

@@ -1,2 +0,0 @@
-[filtered-html-is-not-container.html]
-  expected: FAIL