Refactor behaved building (#3403)

9 files changed, 284 insertions, 398 deletions

diff --git a/crates/typst/src/foundations/content.rs b/crates/typst/src/foundations/content.rs
index 0d764f11..8c093764 100644
--- a/crates/typst/src/foundations/content.rs
+++ b/crates/typst/src/foundations/content.rs
@@ -14,8 +14,9 @@ use smallvec::smallvec;
 use crate::diag::{SourceResult, StrResult};
 use crate::engine::Engine;
 use crate::foundations::{
-    elem, func, scope, ty, Dict, Element, Fields, IntoValue, Label, NativeElement,
-    Recipe, RecipeIndex, Repr, Selector, Str, Style, StyleChain, Styles, Value,
+    elem, func, scope, ty, Behave, Behaviour, Dict, Element, Fields, IntoValue, Label,
+    NativeElement, Recipe, RecipeIndex, Repr, Selector, Str, Style, StyleChain, Styles,
+    Value,
 };
 use crate::introspection::{Location, Meta, MetaElem};
 use crate::layout::{AlignElem, Alignment, Axes, Length, MoveElem, PadElem, Rel, Sides};
@@ -167,6 +168,12 @@ impl Content {
         self.make_mut().lifecycle.insert(0);
     }
 
+    /// How this element interacts with other elements in a stream.
+    pub fn behaviour(&self) -> Behaviour {
+        self.with::<dyn Behave>()
+            .map_or(Behaviour::Supportive, Behave::behaviour)
+    }
+
     /// Get a field by ID.
     ///
     /// This is the preferred way to access fields. However, you can only use it
@@ -335,7 +342,7 @@ impl Content {
     }
 
     /// Also auto expands sequence of sequences into flat sequence
-    pub fn sequence_recursive_for_each(&self, f: &mut impl FnMut(&Self)) {
+    pub fn sequence_recursive_for_each<'a>(&'a self, f: &mut impl FnMut(&'a Self)) {
         if let Some(children) = self.to_sequence() {
             children.for_each(|c| c.sequence_recursive_for_each(f));
         } else {
diff --git a/crates/typst/src/foundations/element.rs b/crates/typst/src/foundations/element.rs
index 412e3089..b59a16cb 100644
--- a/crates/typst/src/foundations/element.rs
+++ b/crates/typst/src/foundations/element.rs
@@ -1,5 +1,4 @@
 use std::any::TypeId;
-use std::borrow::Cow;
 use std::cmp::Ordering;
 use std::fmt::{self, Debug};
 use std::hash::Hash;
@@ -309,11 +308,7 @@ pub trait Behave {
     /// Whether this weak element is larger than a previous one and thus picked
     /// as the maximum when the levels are the same.
     #[allow(unused_variables)]
-    fn larger(
-        &self,
-        prev: &(Cow<Content>, Behaviour, StyleChain),
-        styles: StyleChain,
-    ) -> bool {
+    fn larger(&self, prev: &(&Content, StyleChain), styles: StyleChain) -> bool {
         false
     }
 }
@@ -336,3 +331,10 @@ pub enum Behaviour {
     /// An element that does not have a visual representation.
     Invisible,
 }
+
+impl Behaviour {
+    /// Whether this of `Weak(_)` variant.
+    pub fn is_weak(self) -> bool {
+        matches!(self, Self::Weak(_))
+    }
+}
diff --git a/crates/typst/src/foundations/styles.rs b/crates/typst/src/foundations/styles.rs
index c6ee3c5e..9656fafb 100644
--- a/crates/typst/src/foundations/styles.rs
+++ b/crates/typst/src/foundations/styles.rs
@@ -1,8 +1,7 @@
 use std::any::{Any, TypeId};
-use std::borrow::Cow;
 use std::fmt::{self, Debug, Formatter};
 use std::hash::{Hash, Hasher};
-use std::{iter, mem, ptr};
+use std::{mem, ptr};
 
 use comemo::Prehashed;
 use ecow::{eco_vec, EcoString, EcoVec};
@@ -570,13 +569,13 @@ impl<'a> StyleChain<'a> {
     }
 
     /// Iterate over the links of the chain.
-    fn links(self) -> Links<'a> {
+    pub fn links(self) -> Links<'a> {
         Links(Some(self))
     }
 
     /// Build owned styles from the suffix (all links beyond the `len`) of the
     /// chain.
-    fn suffix(self, len: usize) -> Styles {
+    pub fn suffix(self, len: usize) -> Styles {
         let mut suffix = Styles::new();
         let take = self.links().count().saturating_sub(len);
         for link in self.links().take(take) {
@@ -586,7 +585,7 @@ impl<'a> StyleChain<'a> {
     }
 
     /// Remove the last link from the chain.
-    fn pop(&mut self) {
+    pub fn pop(&mut self) {
         *self = self.tail.copied().unwrap_or_default();
     }
 }
@@ -672,7 +671,7 @@ impl<'a> Iterator for Entries<'a> {
 }
 
 /// An iterator over the links of a style chain.
-struct Links<'a>(Option<StyleChain<'a>>);
+pub struct Links<'a>(Option<StyleChain<'a>>);
 
 impl<'a> Iterator for Links<'a> {
     type Item = &'a [Prehashed<Style>];
@@ -684,201 +683,6 @@ impl<'a> Iterator for Links<'a> {
     }
 }
 
-/// A sequence of items with associated styles.
-#[derive(Clone, Hash)]
-pub struct StyleVec<T> {
-    items: Vec<T>,
-    styles: Vec<(Styles, usize)>,
-}
-
-impl<T> StyleVec<T> {
-    /// Whether there are any items in the sequence.
-    pub fn is_empty(&self) -> bool {
-        self.items.is_empty()
-    }
-
-    /// Number of items in the sequence.
-    pub fn len(&self) -> usize {
-        self.items.len()
-    }
-
-    /// Insert an item in the front. The item will share the style of the
-    /// current first item.
-    ///
-    /// This method has no effect if the vector is empty.
-    pub fn push_front(&mut self, item: T) {
-        if !self.styles.is_empty() {
-            self.items.insert(0, item);
-            self.styles[0].1 += 1;
-        }
-    }
-
-    /// Map the contained items.
-    pub fn map<F, U>(&self, f: F) -> StyleVec<U>
-    where
-        F: FnMut(&T) -> U,
-    {
-        StyleVec {
-            items: self.items.iter().map(f).collect(),
-            styles: self.styles.clone(),
-        }
-    }
-
-    /// Iterate over references to the contained items and associated styles.
-    pub fn iter(&self) -> impl Iterator<Item = (&T, &Styles)> + '_ {
-        self.items().zip(
-            self.styles
-                .iter()
-                .flat_map(|(map, count)| iter::repeat(map).take(*count)),
-        )
-    }
-
-    /// Iterate over the contained items.
-    pub fn items(&self) -> std::slice::Iter<'_, T> {
-        self.items.iter()
-    }
-
-    /// Extract the contained items.
-    pub fn into_items(self) -> Vec<T> {
-        self.items
-    }
-
-    /// Iterate over the contained style lists. Note that zipping this with
-    /// `items()` does not yield the same result as calling `iter()` because
-    /// this method only returns lists once that are shared by consecutive
-    /// items. This method is designed for use cases where you want to check,
-    /// for example, whether any of the lists fulfills a specific property.
-    pub fn styles(&self) -> impl Iterator<Item = &Styles> {
-        self.styles.iter().map(|(map, _)| map)
-    }
-}
-
-impl<'a> StyleVec<Cow<'a, Content>> {
-    pub fn to_vec<F: From<Content>>(self) -> Vec<F> {
-        self.items
-            .into_iter()
-            .zip(
-                self.styles
-                    .iter()
-                    .flat_map(|(map, count)| iter::repeat(map).take(*count)),
-            )
-            .map(|(content, styles)| content.into_owned().styled_with_map(styles.clone()))
-            .map(F::from)
-            .collect()
-    }
-}
-
-impl<T> Default for StyleVec<T> {
-    fn default() -> Self {
-        Self { items: vec![], styles: vec![] }
-    }
-}
-
-impl<T> FromIterator<T> for StyleVec<T> {
-    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
-        let items: Vec<_> = iter.into_iter().collect();
-        let styles = vec![(Styles::new(), items.len())];
-        Self { items, styles }
-    }
-}
-
-impl<T: Debug> Debug for StyleVec<T> {
-    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        f.debug_list()
-            .entries(self.iter().map(|(item, styles)| {
-                crate::util::debug(|f| {
-                    styles.fmt(f)?;
-                    item.fmt(f)
-                })
-            }))
-            .finish()
-    }
-}
-
-/// Assists in the construction of a [`StyleVec`].
-#[derive(Debug)]
-pub struct StyleVecBuilder<'a, T> {
-    items: Vec<T>,
-    chains: Vec<(StyleChain<'a>, usize)>,
-}
-
-impl<'a, T> StyleVecBuilder<'a, T> {
-    /// Create a new style-vec builder.
-    pub fn new() -> Self {
-        Self { items: vec![], chains: vec![] }
-    }
-
-    /// Whether the builder is empty.
-    pub fn is_empty(&self) -> bool {
-        self.items.is_empty()
-    }
-
-    /// Push a new item into the style vector.
-    pub fn push(&mut self, item: T, styles: StyleChain<'a>) {
-        self.items.push(item);
-
-        if let Some((prev, count)) = self.chains.last_mut() {
-            if *prev == styles {
-                *count += 1;
-                return;
-            }
-        }
-
-        self.chains.push((styles, 1));
-    }
-
-    /// Iterate over the contained items.
-    pub fn elems(&self) -> std::slice::Iter<'_, T> {
-        self.items.iter()
-    }
-
-    /// Finish building, returning a pair of two things:
-    /// - a style vector of items with the non-shared styles
-    /// - a shared prefix chain of styles that apply to all items
-    pub fn finish(self) -> (StyleVec<T>, StyleChain<'a>) {
-        let mut iter = self.chains.iter();
-        let mut trunk = match iter.next() {
-            Some(&(chain, _)) => chain,
-            None => return Default::default(),
-        };
-
-        let mut shared = trunk.links().count();
-        for &(mut chain, _) in iter {
-            let len = chain.links().count();
-            if len < shared {
-                for _ in 0..shared - len {
-                    trunk.pop();
-                }
-                shared = len;
-            } else if len > shared {
-                for _ in 0..len - shared {
-                    chain.pop();
-                }
-            }
-
-            while shared > 0 && chain != trunk {
-                trunk.pop();
-                chain.pop();
-                shared -= 1;
-            }
-        }
-
-        let styles = self
-            .chains
-            .into_iter()
-            .map(|(chain, count)| (chain.suffix(shared), count))
-            .collect();
-
-        (StyleVec { items: self.items, styles }, trunk)
-    }
-}
-
-impl<'a, T> Default for StyleVecBuilder<'a, T> {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
 /// A property that is resolved with other properties from the style chain.
 pub trait Resolve {
     /// The type of the resolved output.
diff --git a/crates/typst/src/layout/mod.rs b/crates/typst/src/layout/mod.rs
index c961b693..7f0c3916 100644
--- a/crates/typst/src/layout/mod.rs
+++ b/crates/typst/src/layout/mod.rs
@@ -77,7 +77,7 @@ use crate::eval::Tracer;
 use crate::foundations::{category, Category, Content, Scope, StyleChain};
 use crate::introspection::{Introspector, Locator};
 use crate::model::Document;
-use crate::realize::{realize_block, realize_root, Scratch};
+use crate::realize::{realize_block, realize_root, Arenas};
 use crate::World;
 
 /// Arranging elements on the page in different ways.
@@ -195,9 +195,8 @@ impl LayoutRoot for Content {
                 locator: &mut locator,
                 tracer,
             };
-            let scratch = Scratch::default();
-            let (document, styles) =
-                realize_root(&mut engine, &scratch, content, styles)?;
+            let arenas = Arenas::default();
+            let (document, styles) = realize_root(&mut engine, &arenas, content, styles)?;
             document.layout_root(&mut engine, styles)
         }
 
@@ -248,9 +247,9 @@ impl LayoutMultiple for Content {
                 );
             }
 
-            let scratch = Scratch::default();
+            let arenas = Arenas::default();
             let (realized, styles) =
-                realize_block(&mut engine, &scratch, content, styles)?;
+                realize_block(&mut engine, &arenas, content, styles)?;
             realized.with::<dyn LayoutMultiple>().unwrap().layout(
                 &mut engine,
                 styles,
diff --git a/crates/typst/src/layout/spacing.rs b/crates/typst/src/layout/spacing.rs
index ec029ecd..d6a1d592 100644
--- a/crates/typst/src/layout/spacing.rs
+++ b/crates/typst/src/layout/spacing.rs
@@ -1,5 +1,3 @@
-use std::borrow::Cow;
-
 use crate::foundations::{
     cast, elem, Behave, Behaviour, Content, Packed, Resolve, StyleChain,
 };
@@ -75,16 +73,12 @@ impl Behave for Packed<HElem> {
         }
     }
 
-    fn larger(
-        &self,
-        prev: &(Cow<Content>, Behaviour, StyleChain),
-        styles: StyleChain,
-    ) -> bool {
+    fn larger(&self, prev: &(&Content, StyleChain), styles: StyleChain) -> bool {
         let Some(other) = prev.0.to_packed::<HElem>() else { return false };
         match (self.amount(), other.amount()) {
             (Spacing::Fr(this), Spacing::Fr(other)) => this > other,
             (Spacing::Rel(this), Spacing::Rel(other)) => {
-                this.resolve(styles) > other.resolve(prev.2)
+                this.resolve(styles) > other.resolve(prev.1)
             }
             _ => false,
         }
@@ -177,16 +171,12 @@ impl Behave for Packed<VElem> {
         }
     }
 
-    fn larger(
-        &self,
-        prev: &(Cow<Content>, Behaviour, StyleChain),
-        styles: StyleChain,
-    ) -> bool {
+    fn larger(&self, prev: &(&Content, StyleChain), styles: StyleChain) -> bool {
         let Some(other) = prev.0.to_packed::<VElem>() else { return false };
         match (self.amount(), other.amount()) {
             (Spacing::Fr(this), Spacing::Fr(other)) => this > other,
             (Spacing::Rel(this), Spacing::Rel(other)) => {
-                this.resolve(styles) > other.resolve(prev.2)
+                this.resolve(styles) > other.resolve(prev.1)
             }
             _ => false,
         }
diff --git a/crates/typst/src/math/mod.rs b/crates/typst/src/math/mod.rs
index 3338fb63..63333d0f 100644
--- a/crates/typst/src/math/mod.rs
+++ b/crates/typst/src/math/mod.rs
@@ -40,8 +40,6 @@ use self::fragment::*;
 use self::row::*;
 use self::spacing::*;
 
-use std::borrow::Cow;
-
 use crate::diag::SourceResult;
 use crate::foundations::{
     category, Category, Content, Module, Resolve, Scope, StyleChain,
@@ -239,10 +237,9 @@ impl LayoutMath for Content {
         if self.is_sequence() {
             let mut bb = BehavedBuilder::new();
             self.sequence_recursive_for_each(&mut |child: &Content| {
-                bb.push(Cow::Owned(child.clone()), StyleChain::default())
+                bb.push(child, StyleChain::default());
             });
-
-            for (child, _) in bb.finish().0.iter() {
+            for child in bb.finish::<Content>().0 {
                 child.layout_math(ctx, styles)?;
             }
             return Ok(());
diff --git a/crates/typst/src/realize/arenas.rs b/crates/typst/src/realize/arenas.rs
new file mode 100644
index 00000000..9c90091c
--- /dev/null
+++ b/crates/typst/src/realize/arenas.rs
@@ -0,0 +1,34 @@
+use typed_arena::Arena;
+
+use crate::foundations::{Content, StyleChain};
+
+/// Temporary storage arenas for building.
+#[derive(Default)]
+pub struct Arenas<'a> {
+    chains: Arena<StyleChain<'a>>,
+    content: Arena<Content>,
+}
+
+impl<'a> Arenas<'a> {
+    /// Store a value in the matching arena.
+    pub fn store<T: Store<'a>>(&'a self, val: T) -> &'a T {
+        val.store(self)
+    }
+}
+
+/// Implemented by storable types.
+pub trait Store<'a> {
+    fn store(self, arenas: &'a Arenas<'a>) -> &'a Self;
+}
+
+impl<'a> Store<'a> for Content {
+    fn store(self, arenas: &'a Arenas<'a>) -> &'a Self {
+        arenas.content.alloc(self)
+    }
+}
+
+impl<'a> Store<'a> for StyleChain<'a> {
+    fn store(self, arenas: &'a Arenas<'a>) -> &'a Self {
+        arenas.chains.alloc(self)
+    }
+}
diff --git a/crates/typst/src/realize/behave.rs b/crates/typst/src/realize/behave.rs
index bc900994..035f6644 100644
--- a/crates/typst/src/realize/behave.rs
+++ b/crates/typst/src/realize/behave.rs
@@ -1,107 +1,185 @@
 //! Element interaction.
 
-use std::borrow::Cow;
+use crate::foundations::{Behave, Behaviour, Content, StyleChain, Styles};
+use crate::syntax::Span;
 
-use crate::foundations::{
-    Behave, Behaviour, Content, StyleChain, StyleVec, StyleVecBuilder,
-};
-
-/// A wrapper around a [`StyleVecBuilder`] that allows elements to interact.
+/// Processes a sequence of content and resolves behaviour interactions between
+/// them and separates local styles for each element from the shared trunk of
+/// styles.
 #[derive(Debug)]
 pub struct BehavedBuilder<'a> {
-    /// The internal builder.
-    builder: StyleVecBuilder<'a, Cow<'a, Content>>,
-    /// Staged weak and ignorant elements that we can't yet commit to the
-    /// builder. The option is `Some(_)` for weak elements and `None` for
-    /// ignorant elements.
-    staged: Vec<(Cow<'a, Content>, Behaviour, StyleChain<'a>)>,
-    /// What the last non-ignorant item was.
+    /// The collected content with its styles.
+    buf: Vec<(&'a Content, StyleChain<'a>)>,
+    /// What the last non-ignorant, visible item was.
     last: Behaviour,
 }
 
 impl<'a> BehavedBuilder<'a> {
     /// Create a new style-vec builder.
     pub fn new() -> Self {
-        Self {
-            builder: StyleVecBuilder::new(),
-            staged: vec![],
-            last: Behaviour::Destructive,
-        }
+        Self { buf: vec![], last: Behaviour::Destructive }
     }
 
     /// Whether the builder is totally empty.
     pub fn is_empty(&self) -> bool {
-        self.builder.is_empty() && self.staged.is_empty()
+        self.buf.is_empty()
     }
 
-    /// Whether the builder is empty except for some weak elements that will
-    /// probably collapse.
+    /// Whether the builder has any proper (non-weak & visible) elements.
     pub fn has_strong_elements(&self, last: bool) -> bool {
-        !self.builder.is_empty()
-            || self.staged.iter().any(|(_, behaviour, _)| {
-                !matches!(behaviour, Behaviour::Weak(_) | Behaviour::Invisible)
-                    || (last && *behaviour == Behaviour::Invisible)
-            })
+        self.buf.iter().any(|(content, _)| {
+            let behaviour = content.behaviour();
+            !matches!(behaviour, Behaviour::Weak(_) | Behaviour::Invisible)
+                || (last && behaviour == Behaviour::Invisible)
+        })
     }
 
-    /// Push an item into the sequence.
-    pub fn push(&mut self, elem: Cow<'a, Content>, styles: StyleChain<'a>) {
-        let interaction = elem
-            .with::<dyn Behave>()
-            .map_or(Behaviour::Supportive, Behave::behaviour);
-
-        match interaction {
-            Behaviour::Weak(level) => {
-                if matches!(self.last, Behaviour::Weak(_)) {
-                    let item = elem.with::<dyn Behave>().unwrap();
-                    let i = self.staged.iter().position(|prev| {
-                        let Behaviour::Weak(prev_level) = prev.1 else { return false };
-                        level < prev_level
-                            || (level == prev_level && item.larger(prev, styles))
-                    });
-                    let Some(i) = i else { return };
-                    self.staged.remove(i);
-                }
+    /// Push an item into the builder.
+    pub fn push(&mut self, content: &'a Content, styles: StyleChain<'a>) {
+        let mut behaviour = content.behaviour();
+        match behaviour {
+            Behaviour::Supportive => {}
+            Behaviour::Weak(level) => match self.last {
+                // Remove either this or the preceding weak item.
+                Behaviour::Weak(prev_level) => {
+                    if level > prev_level {
+                        return;
+                    }
 
-                if self.last != Behaviour::Destructive {
-                    self.staged.push((elem, interaction, styles));
-                    self.last = interaction;
+                    let i = self.find_last_weak().unwrap();
+                    if level == prev_level
+                        && !content
+                            .with::<dyn Behave>()
+                            .unwrap()
+                            .larger(&self.buf[i], styles)
+                    {
+                        return;
+                    }
+
+                    self.buf.remove(i);
                 }
-            }
-            Behaviour::Supportive => {
-                self.flush(true);
-                self.builder.push(elem, styles);
-                self.last = interaction;
-            }
+                Behaviour::Destructive => return,
+                _ => {}
+            },
             Behaviour::Destructive => {
-                self.flush(false);
-                self.builder.push(elem, styles);
-                self.last = interaction;
+                // Remove preceding weak item.
+                if self.last.is_weak() {
+                    let i = self.find_last_weak().unwrap();
+                    self.buf.remove(i);
+                }
             }
             Behaviour::Ignorant | Behaviour::Invisible => {
-                self.staged.push((elem, interaction, styles));
+                behaviour = self.last;
             }
         }
+
+        self.last = behaviour;
+        self.buf.push((content, styles));
+    }
+
+    /// Iterate over the content that has been pushed so far.
+    pub fn items(&self) -> impl Iterator<Item = &'a Content> + '_ {
+        self.buf.iter().map(|&(c, _)| c)
     }
 
-    /// Return the finish style vec and the common prefix chain.
-    pub fn finish(mut self) -> (StyleVec<Cow<'a, Content>>, StyleChain<'a>) {
-        self.flush(false);
-        self.builder.finish()
+    /// Return the built content (possibly styled with local styles) plus a
+    /// trunk style chain and a span for the collection.
+    pub fn finish<F: From<Content>>(self) -> (Vec<F>, StyleChain<'a>, Span) {
+        let (output, trunk, span) = self.finish_iter();
+        let output = output.map(|(c, s)| c.clone().styled_with_map(s).into()).collect();
+        (output, trunk, span)
     }
 
-    /// Push the staged elements, filtering out weak elements if `supportive` is
-    /// false.
-    fn flush(&mut self, supportive: bool) {
-        for (item, interaction, styles) in self.staged.drain(..) {
-            if supportive
-                || interaction == Behaviour::Ignorant
-                || interaction == Behaviour::Invisible
-            {
-                self.builder.push(item, styles);
+    /// Return an iterator over the built content and its local styles plus a
+    /// trunk style chain and a span for the collection.
+    pub fn finish_iter(
+        mut self,
+    ) -> (impl Iterator<Item = (&'a Content, Styles)>, StyleChain<'a>, Span) {
+        self.trim_weak();
+
+        let span = self.determine_span();
+        let (trunk, depth) = self.determine_style_trunk();
+
+        let mut iter = self.buf.into_iter().peekable();
+        let mut reuse = None;
+
+        // Map the content + style chains to content + suffix maps, reusing
+        // equivalent adjacent suffix maps, if possible.
+        let output = std::iter::from_fn(move || {
+            let (c, s) = iter.next()?;
+
+            // Try to reuse a suffix map that the previous element has
+            // stored for us.
+            let suffix = reuse.take().unwrap_or_else(|| s.suffix(depth));
+
+            // Store a suffix map for the next element if it has the same style
+            // chain.
+            if iter.peek().map_or(false, |&(_, s2)| s == s2) {
+                reuse = Some(suffix.clone());
             }
+
+            Some((c, suffix))
+        });
+
+        (output, trunk, span)
+    }
+
+    /// Trim a possibly remaining weak item.
+    fn trim_weak(&mut self) {
+        if self.last.is_weak() {
+            let i = self.find_last_weak().unwrap();
+            self.buf.remove(i);
         }
     }
+
+    /// Get the position of the right most weak item.
+    fn find_last_weak(&self) -> Option<usize> {
+        self.buf.iter().rposition(|(c, _)| c.behaviour().is_weak())
+    }
+
+    /// Determine a span for the built collection.
+    fn determine_span(&self) -> Span {
+        let mut span = Span::detached();
+        for &(content, _) in &self.buf {
+            span = content.span();
+            if !span.is_detached() {
+                break;
+            }
+        }
+        span
+    }
+
+    /// Determine the shared trunk style chain.
+    fn determine_style_trunk(&self) -> (StyleChain<'a>, usize) {
+        // Determine shared style depth and first span.
+        let mut trunk = match self.buf.first() {
+            Some(&(_, chain)) => chain,
+            None => Default::default(),
+        };
+
+        let mut depth = trunk.links().count();
+        for (_, mut chain) in &self.buf {
+            let len = chain.links().count();
+            if len < depth {
+                for _ in 0..depth - len {
+                    trunk.pop();
+                }
+                depth = len;
+            } else if len > depth {
+                for _ in 0..len - depth {
+                    chain.pop();
+                }
+            }
+
+            while depth > 0 && chain != trunk {
+                trunk.pop();
+                chain.pop();
+                depth -= 1;
+            }
+        }
+
+        (trunk, depth)
+    }
 }
 
 impl<'a> Default for BehavedBuilder<'a> {
diff --git a/crates/typst/src/realize/mod.rs b/crates/typst/src/realize/mod.rs
index 191d9eed..a03a209e 100644
--- a/crates/typst/src/realize/mod.rs
+++ b/crates/typst/src/realize/mod.rs
@@ -1,7 +1,9 @@
 //! Realization of content.
 
+mod arenas;
 mod behave;
 
+pub use self::arenas::Arenas;
 pub use self::behave::BehavedBuilder;
 
 use std::borrow::Cow;
@@ -9,14 +11,12 @@ use std::cell::OnceCell;
 use std::mem;
 
 use smallvec::smallvec;
-use typed_arena::Arena;
 
 use crate::diag::{bail, SourceResult};
 use crate::engine::{Engine, Route};
 use crate::foundations::{
-    Behave, Behaviour, Content, NativeElement, Packed, Recipe, RecipeIndex, Regex,
-    Selector, Show, ShowSet, Style, StyleChain, StyleVec, StyleVecBuilder, Styles,
-    Synthesize, Transformation,
+    Content, NativeElement, Packed, Recipe, RecipeIndex, Regex, Selector, Show, ShowSet,
+    Style, StyleChain, Styles, Synthesize, Transformation,
 };
 use crate::introspection::{Locatable, Meta, MetaElem};
 use crate::layout::{
@@ -36,15 +36,14 @@ use crate::util::{hash128, BitSet};
 #[typst_macros::time(name = "realize root")]
 pub fn realize_root<'a>(
     engine: &mut Engine,
-    scratch: &'a Scratch<'a>,
+    arenas: &'a Arenas<'a>,
     content: &'a Content,
     styles: StyleChain<'a>,
 ) -> SourceResult<(Packed<DocumentElem>, StyleChain<'a>)> {
-    let mut builder = Builder::new(engine, scratch, true);
+    let mut builder = Builder::new(engine, arenas, true);
     builder.accept(content, styles)?;
     builder.interrupt_page(Some(styles), true)?;
-    let (pages, shared) = builder.doc.unwrap().pages.finish();
-    let span = first_span(&pages);
+    let (pages, shared, span) = builder.doc.unwrap().pages.finish();
     Ok((Packed::new(DocumentElem::new(pages.to_vec())).spanned(span), shared))
 }
 
@@ -52,7 +51,7 @@ pub fn realize_root<'a>(
 #[typst_macros::time(name = "realize block")]
 pub fn realize_block<'a>(
     engine: &mut Engine,
-    scratch: &'a Scratch<'a>,
+    arenas: &'a Arenas<'a>,
     content: &'a Content,
     styles: StyleChain<'a>,
 ) -> SourceResult<(Cow<'a, Content>, StyleChain<'a>)> {
@@ -62,13 +61,12 @@ pub fn realize_block<'a>(
         return Ok((Cow::Borrowed(content), styles));
     }
 
-    let mut builder = Builder::new(engine, scratch, false);
+    let mut builder = Builder::new(engine, arenas, false);
     builder.accept(content, styles)?;
     builder.interrupt_par()?;
 
-    let (children, shared) = builder.flow.0.finish();
-    let span = first_span(&children);
-    Ok((Cow::Owned(FlowElem::new(children.to_vec()).pack().spanned(span)), shared))
+    let (children, shared, span) = builder.flow.0.finish();
+    Ok((Cow::Owned(FlowElem::new(children).pack().spanned(span)), shared))
 }
 
 /// Apply the show rules in the given style chain to a target element.
@@ -366,7 +364,7 @@ struct Builder<'a, 'v, 't> {
     /// The engine.
     engine: &'v mut Engine<'t>,
     /// Scratch arenas for building.
-    scratch: &'a Scratch<'a>,
+    arenas: &'a Arenas<'a>,
     /// The current document building state.
     doc: Option<DocBuilder<'a>>,
     /// The current flow building state.
@@ -379,20 +377,11 @@ struct Builder<'a, 'v, 't> {
     cites: CiteGroupBuilder<'a>,
 }
 
-/// Temporary storage arenas for building.
-#[derive(Default)]
-pub struct Scratch<'a> {
-    /// An arena where intermediate style chains are stored.
-    styles: Arena<StyleChain<'a>>,
-    /// An arena where intermediate content resulting from show rules is stored.
-    content: Arena<Content>,
-}
-
 impl<'a, 'v, 't> Builder<'a, 'v, 't> {
-    fn new(engine: &'v mut Engine<'t>, scratch: &'a Scratch<'a>, top: bool) -> Self {
+    fn new(engine: &'v mut Engine<'t>, arenas: &'a Arenas<'a>, top: bool) -> Self {
         Self {
             engine,
-            scratch,
+            arenas,
             doc: top.then(DocBuilder::default),
             flow: FlowBuilder::default(),
             par: ParBuilder::default(),
@@ -408,9 +397,8 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
     ) -> SourceResult<()> {
         if content.can::<dyn LayoutMath>() && !content.is::<EquationElem>() {
             content = self
-                .scratch
-                .content
-                .alloc(EquationElem::new(content.clone()).pack().spanned(content.span()));
+                .arenas
+                .store(EquationElem::new(content.clone()).pack().spanned(content.span()));
         }
 
         if let Some(realized) = realize(self.engine, content, styles)? {
@@ -421,10 +409,9 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
                     hint: "check whether the show rule matches its own output"
                 );
             }
-            let stored = self.scratch.content.alloc(realized);
-            let v = self.accept(stored, styles);
+            let result = self.accept(self.arenas.store(realized), styles);
             self.engine.route.decrease();
-            return v;
+            return result;
         }
 
         if let Some((elem, local)) = content.to_styled() {
@@ -460,7 +447,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
 
         self.interrupt_par()?;
 
-        if self.flow.accept(content, styles) {
+        if self.flow.accept(self.arenas, content, styles) {
             return Ok(());
         }
 
@@ -471,7 +458,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         self.interrupt_page(keep.then_some(styles), false)?;
 
         if let Some(doc) = &mut self.doc {
-            if doc.accept(content, styles) {
+            if doc.accept(self.arenas, content, styles) {
                 return Ok(());
             }
         }
@@ -489,7 +476,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         map: &'a Styles,
         styles: StyleChain<'a>,
     ) -> SourceResult<()> {
-        let stored = self.scratch.styles.alloc(styles);
+        let stored = self.arenas.store(styles);
         let styles = stored.chain(map);
         self.interrupt_style(map, None)?;
         self.accept(elem, styles)?;
@@ -535,8 +522,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         if !self.cites.items.is_empty() {
             let staged = mem::take(&mut self.cites.staged);
             let (group, styles) = mem::take(&mut self.cites).finish();
-            let stored = self.scratch.content.alloc(group);
-            self.accept(stored, styles)?;
+            self.accept(self.arenas.store(group), styles)?;
             for (content, styles) in staged {
                 self.accept(content, styles)?;
             }
@@ -549,8 +535,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         if !self.list.items.is_empty() {
             let staged = mem::take(&mut self.list.staged);
             let (list, styles) = mem::take(&mut self.list).finish();
-            let stored = self.scratch.content.alloc(list);
-            self.accept(stored, styles)?;
+            self.accept(self.arenas.store(list), styles)?;
             for (content, styles) in staged {
                 self.accept(content, styles)?;
             }
@@ -562,8 +547,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         self.interrupt_list()?;
         if !self.par.0.is_empty() {
             let (par, styles) = mem::take(&mut self.par).finish();
-            let stored = self.scratch.content.alloc(par);
-            self.accept(stored, styles)?;
+            self.accept(self.arenas.store(par), styles)?;
         }
 
         Ok(())
@@ -577,17 +561,15 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
         self.interrupt_par()?;
         let Some(doc) = &mut self.doc else { return Ok(()) };
         if (doc.keep_next && styles.is_some()) || self.flow.0.has_strong_elements(last) {
-            let (children, shared) = mem::take(&mut self.flow).0.finish();
+            let (children, shared, span) = mem::take(&mut self.flow).0.finish();
             let styles = if shared == StyleChain::default() {
                 styles.unwrap_or_default()
             } else {
                 shared
             };
-            let span = first_span(&children);
-            let flow = FlowElem::new(children.to_vec());
+            let flow = FlowElem::new(children);
             let page = PageElem::new(flow.pack().spanned(span));
-            let stored = self.scratch.content.alloc(page.pack().spanned(span));
-            self.accept(stored, styles)?;
+            self.accept(self.arenas.store(page.pack().spanned(span)), styles)?;
         }
         Ok(())
     }
@@ -596,7 +578,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
 /// Accepts pagebreaks and pages.
 struct DocBuilder<'a> {
     /// The page runs built so far.
-    pages: StyleVecBuilder<'a, Cow<'a, Content>>,
+    pages: BehavedBuilder<'a>,
     /// Whether to keep a following page even if it is empty.
     keep_next: bool,
     /// Whether the next page should be cleared to an even or odd number.
@@ -604,7 +586,12 @@ struct DocBuilder<'a> {
 }
 
 impl<'a> DocBuilder<'a> {
-    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
+    fn accept(
+        &mut self,
+        arenas: &'a Arenas<'a>,
+        content: &'a Content,
+        styles: StyleChain<'a>,
+    ) -> bool {
         if let Some(pagebreak) = content.to_packed::<PagebreakElem>() {
             self.keep_next = !pagebreak.weak(styles);
             self.clear_next = pagebreak.to(styles);
@@ -615,9 +602,9 @@ impl<'a> DocBuilder<'a> {
             let elem = if let Some(clear_to) = self.clear_next.take() {
                 let mut page = page.clone();
                 page.push_clear_to(Some(clear_to));
-                Cow::Owned(page.pack())
+                arenas.store(page.pack())
             } else {
-                Cow::Borrowed(content)
+                content
             };
 
             self.pages.push(elem, styles);
@@ -632,7 +619,7 @@ impl<'a> DocBuilder<'a> {
 impl Default for DocBuilder<'_> {
     fn default() -> Self {
         Self {
-            pages: StyleVecBuilder::new(),
+            pages: BehavedBuilder::new(),
             keep_next: true,
             clear_next: None,
         }
@@ -644,7 +631,12 @@ impl Default for DocBuilder<'_> {
 struct FlowBuilder<'a>(BehavedBuilder<'a>, bool);
 
 impl<'a> FlowBuilder<'a> {
-    fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
+    fn accept(
+        &mut self,
+        arenas: &'a Arenas<'a>,
+        content: &'a Content,
+        styles: StyleChain<'a>,
+    ) -> bool {
         if content.is::<ParbreakElem>() {
             self.1 = true;
             return true;
@@ -658,7 +650,7 @@ impl<'a> FlowBuilder<'a> {
             || content.is::<MetaElem>()
             || content.is::<PlaceElem>()
         {
-            self.0.push(Cow::Borrowed(content), styles);
+            self.0.push(content, styles);
             return true;
         }
 
@@ -679,7 +671,7 @@ impl<'a> FlowBuilder<'a> {
             if !last_was_parbreak && is_tight_list {
                 let leading = ParElem::leading_in(styles);
                 let spacing = VElem::list_attach(leading.into());
-                self.0.push(Cow::Owned(spacing.pack()), styles);
+                self.0.push(arenas.store(spacing.pack()), styles);
             }
 
             let (above, below) = if let Some(block) = content.to_packed::<BlockElem>() {
@@ -688,9 +680,9 @@ impl<'a> FlowBuilder<'a> {
                 (BlockElem::above_in(styles), BlockElem::below_in(styles))
             };
 
-            self.0.push(Cow::Owned(above.pack()), styles);
-            self.0.push(Cow::Borrowed(content), styles);
-            self.0.push(Cow::Owned(below.pack()), styles);
+            self.0.push(arenas.store(above.pack()), styles);
+            self.0.push(content, styles);
+            self.0.push(arenas.store(below.pack()), styles);
             return true;
         }
 
@@ -706,7 +698,7 @@ impl<'a> ParBuilder<'a> {
     fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
         if content.is::<MetaElem>() {
             if self.0.has_strong_elements(false) {
-                self.0.push(Cow::Borrowed(content), styles);
+                self.0.push(content, styles);
                 return true;
             }
         } else if content.is::<SpaceElem>()
@@ -719,7 +711,7 @@ impl<'a> ParBuilder<'a> {
                 .map_or(false, |elem| !elem.block(styles))
             || content.is::<BoxElem>()
         {
-            self.0.push(Cow::Borrowed(content), styles);
+            self.0.push(content, styles);
             return true;
         }
 
@@ -727,16 +719,15 @@ impl<'a> ParBuilder<'a> {
     }
 
     fn finish(self) -> (Content, StyleChain<'a>) {
-        let (children, shared) = self.0.finish();
-        let span = first_span(&children);
-        (ParElem::new(children.to_vec()).pack().spanned(span), shared)
+        let (children, shared, span) = self.0.finish();
+        (ParElem::new(children).pack().spanned(span), shared)
     }
 }
 
 /// Accepts list / enum items, spaces, paragraph breaks.
 struct ListBuilder<'a> {
     /// The list items collected so far.
-    items: StyleVecBuilder<'a, Cow<'a, Content>>,
+    items: BehavedBuilder<'a>,
     /// Whether the list contains no paragraph breaks.
     tight: bool,
     /// Trailing content for which it is unclear whether it is part of the list.
@@ -757,11 +748,11 @@ impl<'a> ListBuilder<'a> {
             || content.is::<TermItem>())
             && self
                 .items
-                .elems()
+                .items()
                 .next()
                 .map_or(true, |first| first.func() == content.func())
         {
-            self.items.push(Cow::Borrowed(content), styles);
+            self.items.push(content, styles);
             self.tight &= self.staged.drain(..).all(|(t, _)| !t.is::<ParbreakElem>());
             return true;
         }
@@ -770,53 +761,50 @@ impl<'a> ListBuilder<'a> {
     }
 
     fn finish(self) -> (Content, StyleChain<'a>) {
-        let (items, shared) = self.items.finish();
-        let span = first_span(&items);
-        let item = items.items().next().unwrap();
-        let output = if item.is::<ListItem>() {
+        let (items, shared, span) = self.items.finish_iter();
+        let mut items = items.peekable();
+        let (first, _) = items.peek().unwrap();
+        let output = if first.is::<ListItem>() {
             ListElem::new(
                 items
-                    .iter()
                     .map(|(item, local)| {
                         let mut item = item.to_packed::<ListItem>().unwrap().clone();
-                        let body = item.body().clone().styled_with_map(local.clone());
+                        let body = item.body().clone().styled_with_map(local);
                         item.push_body(body);
                         item
                     })
-                    .collect::<Vec<_>>(),
+                    .collect(),
             )
             .with_tight(self.tight)
             .pack()
             .spanned(span)
-        } else if item.is::<EnumItem>() {
+        } else if first.is::<EnumItem>() {
             EnumElem::new(
                 items
-                    .iter()
                     .map(|(item, local)| {
                         let mut item = item.to_packed::<EnumItem>().unwrap().clone();
-                        let body = item.body().clone().styled_with_map(local.clone());
+                        let body = item.body().clone().styled_with_map(local);
                         item.push_body(body);
                         item
                     })
-                    .collect::<Vec<_>>(),
+                    .collect(),
             )
             .with_tight(self.tight)
             .pack()
             .spanned(span)
-        } else if item.is::<TermItem>() {
+        } else if first.is::<TermItem>() {
             TermsElem::new(
                 items
-                    .iter()
                     .map(|(item, local)| {
                         let mut item = item.to_packed::<TermItem>().unwrap().clone();
                         let term = item.term().clone().styled_with_map(local.clone());
                         let description =
-                            item.description().clone().styled_with_map(local.clone());
+                            item.description().clone().styled_with_map(local);
                         item.push_term(term);
                         item.push_description(description);
                         item
                     })
-                    .collect::<Vec<_>>(),
+                    .collect(),
             )
             .with_tight(self.tight)
             .pack()
@@ -831,7 +819,7 @@ impl<'a> ListBuilder<'a> {
 impl Default for ListBuilder<'_> {
     fn default() -> Self {
         Self {
-            items: StyleVecBuilder::default(),
+            items: BehavedBuilder::default(),
             tight: true,
             staged: vec![],
         }
@@ -875,16 +863,3 @@ impl<'a> CiteGroupBuilder<'a> {
         (CiteGroup::new(self.items).pack().spanned(span), self.styles)
     }
 }
-
-/// Find the first span that isn't detached.
-fn first_span(children: &StyleVec<Cow<Content>>) -> Span {
-    children
-        .iter()
-        .filter(|(elem, _)| {
-            elem.with::<dyn Behave>()
-                .map_or(true, |b| b.behaviour() != Behaviour::Invisible)
-        })
-        .map(|(elem, _)| elem.span())
-        .find(|span| !span.is_detached())
-        .unwrap_or_else(Span::detached)
-}