layout: Implement node geometry queries against BoxTree's Fragment (#36663)

This is a followup to #36629, continuing to implement script-based
layout queries  using the `Fragment`s attached to the `BoxTree`. In this
change, geometry queris (apart from parent offset) are calculated using
`Fragment`s hanging of the `BoxTree`.

In order to make this work, all `Fragment`s for inlines split by blocks,
need to be accessible in the `BoxTree`. This required some changes to
the way that box tree items were stored in DOM `BoxSlot`s. Now every
inline level item can have more than a single `BoxTree` item. These are
carefully collected by the `InlineFormattingContextBuilder` -- currently
a bit fragile, but with more documentation.

Testing: There are tests for these changes.

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

components/layout/flow/construct.rs

@@ -458,15 +458,14 @@ where
                     self.propagated_data.without_text_decorations(),
                 ),
             );
-            box_slot.set(LayoutBox::InlineLevel(atomic));
+            box_slot.set(LayoutBox::InlineLevel(vec![atomic]));
             return;
         };
 
         // Otherwise, this is just a normal inline box. Whatever happened before, all we need to do
         // before recurring is to remember this ongoing inline level box.
-        let inline_item = self
-            .inline_formatting_context_builder
-            .start_inline_box(InlineBox::new(info));
+        self.inline_formatting_context_builder
+            .start_inline_box(InlineBox::new(info), None);
 
         if is_list_item {
             if let Some((marker_info, marker_contents)) =
@@ -486,8 +485,14 @@ where
 
         self.finish_anonymous_table_if_needed();
 
-        self.inline_formatting_context_builder.end_inline_box();
-        box_slot.set(LayoutBox::InlineLevel(inline_item));
+        // As we are ending this inline box, during the course of the `traverse()` above, the ongoing
+        // inline formatting context may have been split around block-level elements. In that case,
+        // more than a single inline box tree item may have been produced for this inline-level box.
+        // `InlineFormattingContextBuilder::end_inline_box()` is returning all of those box tree
+        // items.
+        box_slot.set(LayoutBox::InlineLevel(
+            self.inline_formatting_context_builder.end_inline_box(),
+        ));
     }
 
     fn handle_block_level_element(
@@ -574,7 +579,7 @@ where
                     display_inside,
                     contents,
                 ));
-            box_slot.set(LayoutBox::InlineLevel(inline_level_box));
+            box_slot.set(LayoutBox::InlineLevel(vec![inline_level_box]));
             return;
         }
 
@@ -607,7 +612,7 @@ where
                         contents,
                         self.propagated_data,
                     ));
-            box_slot.set(LayoutBox::InlineLevel(inline_level_box));
+            box_slot.set(LayoutBox::InlineLevel(vec![inline_level_box]));
             return;
         }
 

components/layout/flow/inline/construct.rs

@@ -6,6 +6,7 @@ use std::borrow::Cow;
 use std::char::{ToLowercase, ToUppercase};
 
 use icu_segmenter::WordSegmenter;
+use itertools::izip;
 use servo_arc::Arc;
 use style::computed_values::white_space_collapse::T as WhiteSpaceCollapse;
 use style::values::specified::text::TextTransformCase;
@@ -67,6 +68,16 @@ pub(crate) struct InlineFormattingContextBuilder {
     /// When an inline box ends, it's removed from this stack.
     inline_box_stack: Vec<InlineBoxIdentifier>,
 
+    /// Normally, an inline box produces a single box tree [`InlineItem`]. When a block
+    /// element causes an inline box [to be split], it can produce multiple
+    /// [`InlineItem`]s, all inserted into different [`InlineFormattingContext`]s.
+    /// [`Self::block_in_inline_splits`] is responsible for tracking all of these split
+    /// inline box results, so that they can be inserted into the [`crate::dom::BoxSlot`]
+    /// for the DOM element once it has been processed for BoxTree construction.
+    ///
+    /// [to be split]: https://www.w3.org/TR/CSS2/visuren.html#anonymous-block-level
+    block_in_inline_splits: Vec<Vec<ArcRefCell<InlineItem>>>,
+
     /// Whether or not the inline formatting context under construction has any
     /// uncollapsible text content.
     pub has_uncollapsible_text_content: bool,
@@ -162,29 +173,42 @@ impl InlineFormattingContextBuilder {
         inline_level_box
     }
 
-    pub(crate) fn start_inline_box(&mut self, inline_box: InlineBox) -> ArcRefCell<InlineItem> {
+    pub(crate) fn start_inline_box(
+        &mut self,
+        inline_box: InlineBox,
+        block_in_inline_splits: Option<Vec<ArcRefCell<InlineItem>>>,
+    ) {
         self.push_control_character_string(inline_box.base.style.bidi_control_chars().0);
 
         let (identifier, inline_box) = self.inline_boxes.start_inline_box(inline_box);
         let inline_level_box = ArcRefCell::new(InlineItem::StartInlineBox(inline_box));
         self.inline_items.push(inline_level_box.clone());
         self.inline_box_stack.push(identifier);
-        inline_level_box
+
+        let mut block_in_inline_splits = block_in_inline_splits.unwrap_or_default();
+        block_in_inline_splits.push(inline_level_box);
+        self.block_in_inline_splits.push(block_in_inline_splits);
     }
 
-    pub(crate) fn end_inline_box(&mut self) -> ArcRefCell<InlineBox> {
-        let identifier = self.end_inline_box_internal();
+    /// End the ongoing inline box in this [`InlineFormattingContextBuilder`], returning
+    /// shared references to all of the box tree items that were created for it. More than
+    /// a single box tree items may be produced for a single inline box when that inline
+    /// box is split around a block-level element.
+    pub(crate) fn end_inline_box(&mut self) -> Vec<ArcRefCell<InlineItem>> {
+        let (identifier, block_in_inline_splits) = self.end_inline_box_internal();
         let inline_level_box = self.inline_boxes.get(&identifier);
-        inline_level_box.borrow_mut().is_last_fragment = true;
+        {
+            let mut inline_level_box = inline_level_box.borrow_mut();
+            inline_level_box.is_last_split = true;
+            self.push_control_character_string(inline_level_box.base.style.bidi_control_chars().1);
+        }
 
-        self.push_control_character_string(
-            inline_level_box.borrow().base.style.bidi_control_chars().1,
-        );
-
-        inline_level_box
+        block_in_inline_splits.unwrap_or_default()
     }
 
-    fn end_inline_box_internal(&mut self) -> InlineBoxIdentifier {
+    fn end_inline_box_internal(
+        &mut self,
+    ) -> (InlineBoxIdentifier, Option<Vec<ArcRefCell<InlineItem>>>) {
         let identifier = self
             .inline_box_stack
             .pop()
@@ -193,7 +217,12 @@ impl InlineFormattingContextBuilder {
             .push(ArcRefCell::new(InlineItem::EndInlineBox));
 
         self.inline_boxes.end_inline_box(identifier);
-        identifier
+
+        // This might be `None` if this builder has already drained its block-in-inline-splits
+        // into the new builder on the other side of a new block-in-inline split.
+        let block_in_inline_splits = self.block_in_inline_splits.pop();
+
+        (identifier, block_in_inline_splits)
     }
 
     pub(crate) fn push_text<'dom, Node: NodeExt<'dom>>(
@@ -295,12 +324,21 @@ impl InlineFormattingContextBuilder {
         // marked as not being the first fragment. No inline content is carried over to this new
         // builder.
         let mut new_builder = InlineFormattingContextBuilder::new();
-        for identifier in self.inline_box_stack.iter() {
+        let block_in_inline_splits = std::mem::take(&mut self.block_in_inline_splits);
+        for (identifier, historical_inline_boxes) in
+            izip!(self.inline_box_stack.iter(), block_in_inline_splits)
+        {
+            // Start a new inline box for every ongoing inline box in this
+            // InlineFormattingContext once we are done processing this block element,
+            // being sure to give the block-in-inline-split to the new
+            // InlineFormattingContext. These will finally be inserted into the DOM's
+            // BoxSlot once the inline box has been fully processed.
             new_builder.start_inline_box(
                 self.inline_boxes
                     .get(identifier)
                     .borrow()
                     .split_around_block(),
+                Some(historical_inline_boxes),
             );
         }
         let mut inline_builder_from_before_split = std::mem::replace(self, new_builder);

components/layout/flow/inline/inline_box.rs

@@ -23,8 +23,12 @@ pub(crate) struct InlineBox {
     pub base: LayoutBoxBase,
     /// The identifier of this inline box in the containing [`super::InlineFormattingContext`].
     pub(super) identifier: InlineBoxIdentifier,
-    pub is_first_fragment: bool,
-    pub is_last_fragment: bool,
+    /// Whether or not this is the first instance of an [`InlineBox`] before a possible
+    /// block-in-inline split. When no split occurs, this is always true.
+    pub is_first_split: bool,
+    /// Whether or not this is the last instance of an [`InlineBox`] before a possible
+    /// block-in-inline split. When no split occurs, this is always true.
+    pub is_last_split: bool,
     /// The index of the default font in the [`super::InlineFormattingContext`]'s font metrics store.
     /// This is initialized during IFC shaping.
     pub default_font_index: Option<usize>,
@@ -36,8 +40,8 @@ impl InlineBox {
             base: LayoutBoxBase::new(info.into(), info.style.clone()),
             // This will be assigned later, when the box is actually added to the IFC.
             identifier: InlineBoxIdentifier::default(),
-            is_first_fragment: true,
-            is_last_fragment: false,
+            is_first_split: true,
+            is_last_split: false,
             default_font_index: None,
         }
     }
@@ -45,8 +49,8 @@ impl InlineBox {
     pub(crate) fn split_around_block(&self) -> Self {
         Self {
             base: LayoutBoxBase::new(self.base.base_fragment_info, self.base.style.clone()),
-            is_first_fragment: false,
-            is_last_fragment: false,
+            is_first_split: false,
+            is_last_split: false,
             ..*self
         }
     }

components/layout/flow/inline/mod.rs

@@ -744,7 +744,7 @@ impl InlineFormattingContextLayout<'_> {
             self.containing_block,
             self.layout_context,
             self.current_inline_container_state(),
-            inline_box.is_last_fragment,
+            inline_box.is_last_split,
             inline_box
                 .default_font_index
                 .map(|index| &self.ifc.font_metrics[index].metrics),
@@ -773,7 +773,7 @@ impl InlineFormattingContextLayout<'_> {
             );
         }
 
-        if inline_box.is_first_fragment {
+        if inline_box.is_first_split {
             self.current_line_segment.inline_size += inline_box_state.pbm.padding.inline_start +
                 inline_box_state.pbm.border.inline_start +
                 inline_box_state.pbm.margin.inline_start.auto_is(Au::zero);
@@ -2349,10 +2349,10 @@ impl<'layout_data> ContentSizesComputation<'layout_data> {
                     .auto_is(Au::zero);
 
                 let pbm = margin + padding + border;
-                if inline_box.is_first_fragment {
+                if inline_box.is_first_split {
                     self.add_inline_size(pbm.inline_start);
                 }
-                if inline_box.is_last_fragment {
+                if inline_box.is_last_split {
                     self.ending_inline_pbm_stack.push(pbm.inline_end);
                 } else {
                     self.ending_inline_pbm_stack.push(Au::zero());

components/layout/flow/root.rs

@@ -195,16 +195,17 @@ impl BoxTree {
                         },
                         _ => return None,
                     },
-                    LayoutBox::InlineLevel(inline_level_box) => match &*inline_level_box.borrow() {
-                        InlineItem::OutOfFlowAbsolutelyPositionedBox(_, text_offset_index)
-                            if box_style.position.is_absolutely_positioned() =>
-                        {
-                            UpdatePoint::AbsolutelyPositionedInlineLevelBox(
-                                inline_level_box.clone(),
-                                *text_offset_index,
-                            )
-                        },
-                        _ => return None,
+                    LayoutBox::InlineLevel(inline_level_items) => {
+                        let inline_level_box = inline_level_items.first()?;
+                        let InlineItem::OutOfFlowAbsolutelyPositionedBox(_, text_offset_index) =
+                            &*inline_level_box.borrow()
+                        else {
+                            return None;
+                        };
+                        UpdatePoint::AbsolutelyPositionedInlineLevelBox(
+                            inline_level_box.clone(),
+                            *text_offset_index,
+                        )
                     },
                     LayoutBox::FlexLevel(flex_level_box) => match &*flex_level_box.borrow() {
                         FlexLevelBox::OutOfFlowAbsolutelyPositionedBox(_)