layout: Store Fragment results in LayoutBoxBase and start using them for queries (#36583)

Start storing a link to laid-out `Fragment`s in `LayoutBoxBase`, so that
these are accessible for queries and eventually for incremental layout.
Some box tree data structures lacked a `LayoutBoxBase`, such as table
tracks and table track groups[^1].

In addition, start using these `Fragment`s for queries instead of
walking the entire `Fragment` tree. Currently, this isn't possible for
most queries as `Fragment`s do not cache their absolute offsets (which
are often necessary). This change uses the new box tree `Fragment`s for
most resolved style queries.

[^1]: Note that only rows and row groups store `Fragment`s as columsn
and
   colgroups do not produce any.

Testing: This is covered by existing tests.
Fixes: This is part of #36525.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/layout_2020/flexbox/layout.rs

@@ -214,7 +214,7 @@ impl FlexLineItem<'_> {
         flex_context: &mut FlexContext,
         all_baselines: &mut Baselines,
         main_position_cursor: &mut Au,
-    ) -> (BoxFragment, PositioningContext) {
+    ) -> (ArcRefCell<BoxFragment>, PositioningContext) {
         // https://drafts.csswg.org/css-flexbox/#algo-main-align
         // “Align the items along the main-axis”
         *main_position_cursor +=
@@ -312,6 +312,12 @@ impl FlexLineItem<'_> {
                 .to_physical_size(containing_block.style.writing_mode)
         }
 
+        let fragment = ArcRefCell::new(fragment);
+        self.item
+            .box_
+            .independent_formatting_context
+            .base
+            .set_fragment(Fragment::Box(fragment.clone()));
         (fragment, self.layout_result.positioning_context)
     }
 }
@@ -323,7 +329,7 @@ struct FinalFlexLineLayout {
     cross_size: Au,
     /// The [`BoxFragment`]s and [`PositioningContext`]s of all flex items,
     /// one per flex item in "order-modified document order."
-    item_fragments: Vec<(BoxFragment, PositioningContext)>,
+    item_fragments: Vec<(ArcRefCell<BoxFragment>, PositioningContext)>,
     /// The 'shared alignment baseline' of this flex line. This is the baseline used for
     /// baseline-aligned items if there are any, otherwise `None`.
     shared_alignment_baseline: Option<Au>,
@@ -935,7 +941,7 @@ impl FlexContainer {
                 let physical_line_position =
                     flow_relative_line_position.to_physical_size(self.style.writing_mode);
                 for (fragment, _) in &mut final_line_layout.item_fragments {
-                    fragment.content_rect.origin += physical_line_position;
+                    fragment.borrow_mut().content_rect.origin += physical_line_position;
                 }
                 final_line_layout.item_fragments
             })
@@ -957,7 +963,7 @@ impl FlexContainer {
                     // per flex item, in the original order.
                     let (fragment, mut child_positioning_context) =
                         flex_item_fragments.next().unwrap();
-                    let fragment = Fragment::Box(ArcRefCell::new(fragment));
+                    let fragment = Fragment::Box(fragment);
                     child_positioning_context.adjust_static_position_of_hoisted_fragments(
                         &fragment,
                         PositioningContextLength::zero(),

components/layout_2020/flexbox/mod.rs

@@ -19,7 +19,7 @@ use crate::context::LayoutContext;
 use crate::dom::{LayoutBox, NodeExt};
 use crate::dom_traversal::{NodeAndStyleInfo, NonReplacedContents};
 use crate::formatting_contexts::IndependentFormattingContext;
-use crate::fragment_tree::BaseFragmentInfo;
+use crate::fragment_tree::{BaseFragmentInfo, Fragment};
 use crate::positioned::AbsolutelyPositionedBox;
 
 mod geom;
@@ -157,6 +157,18 @@ impl FlexLevelBox {
                 .invalidate_cached_fragment(),
         }
     }
+
+    pub(crate) fn fragments(&self) -> Vec<Fragment> {
+        match self {
+            FlexLevelBox::FlexItem(flex_item_box) => flex_item_box
+                .independent_formatting_context
+                .base
+                .fragments(),
+            FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(positioned_box) => {
+                positioned_box.borrow().context.base.fragments()
+            },
+        }
+    }
 }
 
 pub(crate) struct FlexItemBox {