Flow collection phase (#4931)

Cargo.toml

@@ -33,7 +33,7 @@ arrayvec = "0.7.4"
 az = "1.2"
 base64 = "0.22"
 bitflags = { version = "2", features = ["serde"] }
-bumpalo = { version = "3", features = ["collections"] }
+bumpalo = { version = "3", features = ["boxed", "collections"] }
 bytemuck = "1"
 chinese-number = { version = "0.7.2", default-features = false, features = ["number-to-chinese"] }
 chrono = { version = "0.4.24", default-features = false, features = ["clock", "std"] }

crates/typst/src/layout/container.rs

@@ -470,7 +470,7 @@ pub struct BlockElem {
     /// The contents of the block.
     #[positional]
     #[borrowed]
-    pub body: Option<BlockChild>,
+    pub body: Option<BlockBody>,
 }
 
 impl BlockElem {
@@ -490,7 +490,7 @@ impl BlockElem {
     ) -> Self {
         Self::new()
             .with_breakable(false)
-            .with_body(Some(BlockChild::SingleLayouter(
+            .with_body(Some(BlockBody::SingleLayouter(
                 callbacks::BlockSingleCallback::new(captured, f),
             )))
     }
@@ -506,7 +506,7 @@ impl BlockElem {
             regions: Regions,
         ) -> SourceResult<Fragment>,
     ) -> Self {
-        Self::new().with_body(Some(BlockChild::MultiLayouter(
+        Self::new().with_body(Some(BlockBody::MultiLayouter(
             callbacks::BlockMultiCallback::new(captured, f),
         )))
     }
@@ -555,7 +555,7 @@ impl Packed<BlockElem> {
             }
 
             // If we have content as our body, just layout it.
-            Some(BlockChild::Content(body)) => {
+            Some(BlockBody::Content(body)) => {
                 let mut fragment =
                     layout_fragment(engine, body, locator.relayout(), styles, pod)?;
 
@@ -586,7 +586,7 @@ impl Packed<BlockElem> {
 
             // If we have a child that wants to layout with just access to the
             // base region, give it that.
-            Some(BlockChild::SingleLayouter(callback)) => {
+            Some(BlockBody::SingleLayouter(callback)) => {
                 let pod = Region::new(pod.base(), pod.expand);
                 callback.call(engine, locator, styles, pod).map(Fragment::frame)?
             }
@@ -597,7 +597,7 @@ impl Packed<BlockElem> {
             // For auto-sized multi-layouters, we propagate the outer expansion
             // so that they can decide for themselves. We also ensure again to
             // only expand if the size is finite.
-            Some(BlockChild::MultiLayouter(callback)) => {
+            Some(BlockBody::MultiLayouter(callback)) => {
                 let expand = (pod.expand | regions.expand) & pod.size.map(Abs::is_finite);
                 let pod = Regions { expand, ..pod };
                 callback.call(engine, locator, styles, pod)?
@@ -619,7 +619,7 @@ impl Packed<BlockElem> {
         let clip = self.clip(styles);
         let has_fill_or_stroke = fill.is_some() || stroke.iter().any(Option::is_some);
         let has_inset = !inset.is_zero();
-        let is_explicit = matches!(body, None | Some(BlockChild::Content(_)));
+        let is_explicit = matches!(body, None | Some(BlockBody::Content(_)));
 
         // Skip filling/stroking the first frame if it is empty and a non-empty
         // one follows.
@@ -787,7 +787,7 @@ impl Packed<BlockElem> {
 
 /// The contents of a block.
 #[derive(Debug, Clone, PartialEq, Hash)]
-pub enum BlockChild {
+pub enum BlockBody {
     /// The block contains normal content.
     Content(Content),
     /// The block contains a layout callback that needs access to just one
@@ -798,14 +798,14 @@ pub enum BlockChild {
     MultiLayouter(callbacks::BlockMultiCallback),
 }
 
-impl Default for BlockChild {
+impl Default for BlockBody {
     fn default() -> Self {
         Self::Content(Content::default())
     }
 }
 
 cast! {
-    BlockChild,
+    BlockBody,
     self => match self {
         Self::Content(content) => content.into_value(),
         _ => Value::Auto,

crates/typst/src/layout/flow/collect.rs

@@ -0,0 +1,264 @@
+use bumpalo::boxed::Box as BumpBox;
+use bumpalo::Bump;
+use once_cell::unsync::Lazy;
+
+use crate::diag::{bail, SourceResult};
+use crate::engine::Engine;
+use crate::foundations::{Packed, Resolve, Smart, StyleChain};
+use crate::introspection::{Locator, Tag, TagElem};
+use crate::layout::{
+    layout_frame, Abs, AlignElem, Alignment, Axes, BlockElem, ColbreakElem,
+    FixedAlignment, FlushElem, Fr, Fragment, Frame, PagebreakElem, PlaceElem, Ratio,
+    Region, Regions, Rel, Size, Spacing, VElem,
+};
+use crate::model::ParElem;
+use crate::realize::Pair;
+use crate::text::TextElem;
+
+/// A prepared child in flow layout.
+///
+/// The larger variants are bump-boxed to keep the enum size down.
+pub enum Child<'a> {
+    /// An introspection tag.
+    Tag(&'a Tag),
+    /// Relative spacing with a specific weakness.
+    Rel(Rel<Abs>, u8),
+    /// Fractional spacing.
+    Fr(Fr),
+    /// An already layouted line of a paragraph.
+    Line(BumpBox<'a, LineChild>),
+    /// A potentially breakable block.
+    Block(BumpBox<'a, BlockChild<'a>>),
+    /// An absolutely or floatingly placed element.
+    Placed(BumpBox<'a, PlacedChild<'a>>),
+    /// A column break.
+    Break(bool),
+    /// A place flush.
+    Flush,
+}
+
+/// Collects all content of the flow into prepared children.
+#[typst_macros::time]
+pub fn collect<'a>(
+    engine: &mut Engine,
+    bump: &'a Bump,
+    children: &[Pair<'a>],
+    locator: Locator<'a>,
+    base: Size,
+    expand: bool,
+) -> SourceResult<Vec<Child<'a>>> {
+    let mut locator = locator.split();
+    let mut output = Vec::with_capacity(children.len());
+    let mut last_was_par = false;
+
+    for &(child, styles) in children {
+        if let Some(elem) = child.to_packed::<TagElem>() {
+            output.push(Child::Tag(&elem.tag));
+        } else if let Some(elem) = child.to_packed::<VElem>() {
+            output.push(match elem.amount {
+                Spacing::Rel(rel) => {
+                    Child::Rel(rel.resolve(styles), elem.weak(styles) as u8)
+                }
+                Spacing::Fr(fr) => Child::Fr(fr),
+            });
+        } else if let Some(elem) = child.to_packed::<ColbreakElem>() {
+            output.push(Child::Break(elem.weak(styles)));
+        } else if let Some(elem) = child.to_packed::<ParElem>() {
+            let align = AlignElem::alignment_in(styles).resolve(styles);
+            let leading = ParElem::leading_in(styles);
+            let spacing = ParElem::spacing_in(styles);
+            let costs = TextElem::costs_in(styles);
+
+            let lines = crate::layout::layout_inline(
+                engine,
+                &elem.children,
+                locator.next(&elem.span()),
+                styles,
+                last_was_par,
+                base,
+                expand,
+            )?
+            .into_frames();
+
+            output.push(Child::Rel(spacing.into(), 4));
+
+            // Determine whether to prevent widow and orphans.
+            let len = lines.len();
+            let prevent_orphans =
+                costs.orphan() > Ratio::zero() && len >= 2 && !lines[1].is_empty();
+            let prevent_widows =
+                costs.widow() > Ratio::zero() && len >= 2 && !lines[len - 2].is_empty();
+            let prevent_all = len == 3 && prevent_orphans && prevent_widows;
+
+            // Store the heights of lines at the edges because we'll potentially
+            // need these later when `lines` is already moved.
+            let height_at = |i| lines.get(i).map(Frame::height).unwrap_or_default();
+            let front_1 = height_at(0);
+            let front_2 = height_at(1);
+            let back_2 = height_at(len.saturating_sub(2));
+            let back_1 = height_at(len.saturating_sub(1));
+
+            for (i, frame) in lines.into_iter().enumerate() {
+                if i > 0 {
+                    output.push(Child::Rel(leading.into(), 5));
+                }
+
+                // To prevent widows and orphans, we require enough space for
+                // - all lines if it's just three
+                // - the first two lines if we're at the first line
+                // - the last two lines if we're at the second to last line
+                let need = if prevent_all && i == 0 {
+                    front_1 + leading + front_2 + leading + back_1
+                } else if prevent_orphans && i == 0 {
+                    front_1 + leading + front_2
+                } else if prevent_widows && i >= 2 && i + 2 == len {
+                    back_2 + leading + back_1
+                } else {
+                    frame.height()
+                };
+
+                let child = LineChild { frame, align, need };
+                output.push(Child::Line(BumpBox::new_in(child, bump)));
+            }
+
+            output.push(Child::Rel(spacing.into(), 4));
+            last_was_par = true;
+        } else if let Some(elem) = child.to_packed::<BlockElem>() {
+            let locator = locator.next(&elem.span());
+            let align = AlignElem::alignment_in(styles).resolve(styles);
+            let sticky = elem.sticky(styles);
+            let rootable = elem.rootable(styles);
+
+            let fallback = Lazy::new(|| ParElem::spacing_in(styles));
+            let spacing = |amount| match amount {
+                Smart::Auto => Child::Rel((*fallback).into(), 4),
+                Smart::Custom(Spacing::Rel(rel)) => Child::Rel(rel.resolve(styles), 3),
+                Smart::Custom(Spacing::Fr(fr)) => Child::Fr(fr),
+            };
+
+            output.push(spacing(elem.above(styles)));
+
+            let child = BlockChild { align, sticky, rootable, elem, styles, locator };
+            output.push(Child::Block(BumpBox::new_in(child, bump)));
+
+            output.push(spacing(elem.below(styles)));
+            last_was_par = false;
+        } else if let Some(elem) = child.to_packed::<PlaceElem>() {
+            let locator = locator.next(&elem.span());
+            let float = elem.float(styles);
+            let clearance = elem.clearance(styles);
+            let delta = Axes::new(elem.dx(styles), elem.dy(styles)).resolve(styles);
+
+            let alignment = elem.alignment(styles);
+            let align_x = alignment.map_or(FixedAlignment::Center, |align| {
+                align.x().unwrap_or_default().resolve(styles)
+            });
+            let align_y = alignment.map(|align| align.y().map(|y| y.resolve(styles)));
+
+            match (float, align_y) {
+                (true, Smart::Custom(None | Some(FixedAlignment::Center))) => bail!(
+                    elem.span(),
+                    "floating placement must be `auto`, `top`, or `bottom`"
+                ),
+                (false, Smart::Auto) => bail!(
+                    elem.span(),
+                    "automatic positioning is only available for floating placement";
+                    hint: "you can enable floating placement with `place(float: true, ..)`"
+                ),
+                _ => {}
+            }
+
+            let child = PlacedChild {
+                float,
+                clearance,
+                delta,
+                align_x,
+                align_y,
+                elem,
+                styles,
+                locator,
+                alignment,
+            };
+            output.push(Child::Placed(BumpBox::new_in(child, bump)));
+        } else if child.is::<FlushElem>() {
+            output.push(Child::Flush);
+        } else if child.is::<PagebreakElem>() {
+            bail!(
+                child.span(), "pagebreaks are not allowed inside of containers";
+                hint: "try using a `#colbreak()` instead",
+            );
+        } else {
+            bail!(child.span(), "{} is not allowed here", child.func().name());
+        }
+    }
+
+    Ok(output)
+}
+
+/// A child that encapsulates a paragraph line.
+pub struct LineChild {
+    pub frame: Frame,
+    pub align: Axes<FixedAlignment>,
+    pub need: Abs,
+}
+
+/// A child that encapsulates a prepared block.
+pub struct BlockChild<'a> {
+    pub align: Axes<FixedAlignment>,
+    pub sticky: bool,
+    pub rootable: bool,
+    elem: &'a Packed<BlockElem>,
+    styles: StyleChain<'a>,
+    locator: Locator<'a>,
+}
+
+impl BlockChild<'_> {
+    /// Build the child's frames given regions.
+    pub fn layout(
+        &self,
+        engine: &mut Engine,
+        regions: Regions,
+    ) -> SourceResult<Fragment> {
+        let mut fragment =
+            self.elem
+                .layout(engine, self.locator.relayout(), self.styles, regions)?;
+
+        for frame in &mut fragment {
+            frame.post_process(self.styles);
+        }
+
+        Ok(fragment)
+    }
+}
+
+/// A child that encapsulates a prepared placed element.
+pub struct PlacedChild<'a> {
+    pub float: bool,
+    pub clearance: Abs,
+    pub delta: Axes<Rel<Abs>>,
+    pub align_x: FixedAlignment,
+    pub align_y: Smart<Option<FixedAlignment>>,
+    elem: &'a Packed<PlaceElem>,
+    styles: StyleChain<'a>,
+    locator: Locator<'a>,
+    alignment: Smart<Alignment>,
+}
+
+impl PlacedChild<'_> {
+    /// Build the child's frame given the region's base size.
+    pub fn layout(&self, engine: &mut Engine, base: Size) -> SourceResult<Frame> {
+        let align = self.alignment.unwrap_or_else(|| Alignment::CENTER);
+        let aligned = AlignElem::set_alignment(align).wrap();
+
+        let mut frame = layout_frame(
+            engine,
+            &self.elem.body,
+            self.locator.relayout(),
+            self.styles.chain(&aligned),
+            Region::new(base, Axes::splat(false)),
+        )?;
+
+        frame.post_process(self.styles);
+        Ok(frame)
+    }
+}

crates/typst/src/layout/flow/pages.rs

@@ -0,0 +1,566 @@
+use std::collections::HashSet;
+
+use comemo::{Track, Tracked, TrackedMut};
+
+use super::layout_flow;
+use crate::diag::SourceResult;
+use crate::engine::{Engine, Route, Sink, Traced};
+use crate::foundations::{Content, NativeElement, Resolve, Smart, StyleChain, Styles};
+use crate::introspection::{
+    Counter, CounterDisplayElem, CounterKey, Introspector, Locator, LocatorLink,
+    ManualPageCounter, SplitLocator, Tag, TagElem, TagKind,
+};
+use crate::layout::{
+    layout_frame, Abs, AlignElem, Alignment, Axes, Binding, ColumnsElem, Dir, Frame,
+    FrameItem, HAlignment, Length, OuterVAlignment, Page, PageElem, PagebreakElem, Paper,
+    Parity, Point, Region, Regions, Rel, Sides, Size, VAlignment,
+};
+use crate::model::Numbering;
+use crate::realize::Pair;
+use crate::syntax::Span;
+use crate::text::TextElem;
+use crate::utils::Numeric;
+use crate::visualize::Paint;
+use crate::World;
+
+/// An item in page layout.
+enum PageItem<'a> {
+    /// A page run containing content. All runs will be layouted in parallel.
+    Run(&'a [Pair<'a>], StyleChain<'a>, Locator<'a>),
+    /// Tags in between pages. These will be prepended to the first start of
+    /// the next page, or appended at the very end of the final page if there is
+    /// no next page.
+    Tags(&'a [Pair<'a>]),
+    /// An instruction to possibly add a page to bring the page number parity to
+    /// the desired state. Can only be done at the end, sequentially, because it
+    /// requires knowledge of the concrete page number.
+    Parity(Parity, StyleChain<'a>, Locator<'a>),
+}
+
+/// A mostly finished layout for one page. Needs only knowledge of its exact
+/// page number to be finalized into a `Page`. (Because the margins can depend
+/// on the page number.)
+#[derive(Clone)]
+struct LayoutedPage {
+    inner: Frame,
+    margin: Sides<Abs>,
+    binding: Binding,
+    two_sided: bool,
+    header: Option<Frame>,
+    footer: Option<Frame>,
+    background: Option<Frame>,
+    foreground: Option<Frame>,
+    fill: Smart<Option<Paint>>,
+    numbering: Option<Numbering>,
+}
+
+/// Layouts the document's pages.
+pub fn layout_pages<'a>(
+    engine: &mut Engine,
+    children: &'a mut [Pair<'a>],
+    locator: SplitLocator<'a>,
+    styles: StyleChain<'a>,
+) -> SourceResult<Vec<Page>> {
+    // Slice up the children into logical parts.
+    let items = collect_page_items(children, locator, styles);
+
+    // Layout the page runs in parallel.
+    let mut runs = engine.parallelize(
+        items.iter().filter_map(|item| match item {
+            PageItem::Run(children, initial, locator) => {
+                Some((children, initial, locator.relayout()))
+            }
+            _ => None,
+        }),
+        |engine, (children, initial, locator)| {
+            layout_page_run(engine, children, locator, *initial)
+        },
+    );
+
+    let mut pages = vec![];
+    let mut tags = vec![];
+    let mut counter = ManualPageCounter::new();
+
+    // Collect and finalize the runs, handling things like page parity and tags
+    // between pages.
+    for item in &items {
+        match item {
+            PageItem::Run(..) => {
+                let layouted = runs.next().unwrap()?;
+                for layouted in layouted {
+                    let page = finalize_page(engine, &mut counter, &mut tags, layouted)?;
+                    pages.push(page);
+                }
+            }
+            PageItem::Parity(parity, initial, locator) => {
+                if !parity.matches(pages.len()) {
+                    continue;
+                }
+
+                let layouted = layout_blank_page(engine, locator.relayout(), *initial)?;
+                let page = finalize_page(engine, &mut counter, &mut tags, layouted)?;
+                pages.push(page);
+            }
+            PageItem::Tags(items) => {
+                tags.extend(
+                    items
+                        .iter()
+                        .filter_map(|(c, _)| c.to_packed::<TagElem>())
+                        .map(|elem| elem.tag.clone()),
+                );
+            }
+        }
+    }
+
+    // Add the remaining tags to the very end of the last page.
+    if !tags.is_empty() {
+        let last = pages.last_mut().unwrap();
+        let pos = Point::with_y(last.frame.height());
+        last.frame
+            .push_multiple(tags.into_iter().map(|tag| (pos, FrameItem::Tag(tag))));
+    }
+
+    Ok(pages)
+}
+
+/// Slices up the children into logical parts, processing styles and handling
+/// things like tags and weak pagebreaks.
+fn collect_page_items<'a>(
+    mut children: &'a mut [Pair<'a>],
+    mut locator: SplitLocator<'a>,
+    mut initial: StyleChain<'a>,
+) -> Vec<PageItem<'a>> {
+    // The collected page-level items.
+    let mut items: Vec<PageItem<'a>> = vec![];
+    // When this is true, an empty page should be added to `pages` at the end.
+    let mut staged_empty_page = true;
+
+    // The `children` are a flat list of flow-level items and pagebreaks. This
+    // loops splits it up into pagebreaks and consecutive slices of
+    // non-pagebreaks. From these pieces, we build page items that we can then
+    // layout in parallel.
+    while let Some(&(elem, styles)) = children.first() {
+        if let Some(pagebreak) = elem.to_packed::<PagebreakElem>() {
+            // Add a blank page if we encounter a strong pagebreak and there was
+            // a staged empty page.
+            let strong = !pagebreak.weak(styles);
+            if strong && staged_empty_page {
+                let locator = locator.next(&elem.span());
+                items.push(PageItem::Run(&[], initial, locator));
+            }
+
+            // Add an instruction to adjust the page parity if requested.
+            if let Some(parity) = pagebreak.to(styles) {
+                let locator = locator.next(&elem.span());
+                items.push(PageItem::Parity(parity, styles, locator));
+            }
+
+            // The initial styles for the next page are ours unless this is a
+            // "boundary" pagebreak. Such a pagebreak is generated at the end of
+            // the scope of a page set rule to ensure a page boundary. It's
+            // styles correspond to the styles _before_ the page set rule, so we
+            // don't want to apply it to a potential empty page.
+            if !pagebreak.boundary(styles) {
+                initial = styles;
+            }
+
+            // Stage an empty page after a strong pagebreak.
+            staged_empty_page |= strong;
+
+            // Advance to the next child.
+            children = &mut children[1..];
+        } else {
+            // Find the end of the consecutive non-pagebreak run.
+            let end =
+                children.iter().take_while(|(c, _)| !c.is::<PagebreakElem>()).count();
+
+            // Migrate start tags without accompanying end tags from before a
+            // pagebreak to after it.
+            let end = migrate_unterminated_tags(children, end);
+            if end == 0 {
+                continue;
+            }
+
+            // Advance to the rest of the children.
+            let (group, rest) = children.split_at_mut(end);
+            children = rest;
+
+            // If all that is left now are tags, then we don't want to add a
+            // page just for them (since no group would have been detected in a
+            // tagless layout and tags should never affect the layout). For this
+            // reason, we remember them in a `PageItem::Tags` and later insert
+            // them at the _very start_ of the next page, even before the
+            // header.
+            //
+            // We don't do this if all that's left is end boundary pagebreaks
+            // and if an empty page is still staged, since then we can just
+            // conceptually replace that final page with us.
+            if group.iter().all(|(c, _)| c.is::<TagElem>())
+                && !(staged_empty_page
+                    && children.iter().all(|&(c, s)| {
+                        c.to_packed::<PagebreakElem>().is_some_and(|c| c.boundary(s))
+                    }))
+            {
+                items.push(PageItem::Tags(group));
+                continue;
+            }
+
+            // Record a page run and then disregard a staged empty page because
+            // we have real content now.
+            let locator = locator.next(&elem.span());
+            items.push(PageItem::Run(group, initial, locator));
+            staged_empty_page = false;
+        }
+    }
+
+    // Flush a staged empty page.
+    if staged_empty_page {
+        items.push(PageItem::Run(&[], initial, locator.next(&())));
+    }
+
+    items
+}
+
+/// Migrates trailing start tags without accompanying end tags tags from before
+/// a pagebreak to after it. Returns the position right after the last
+/// non-migrated tag.
+///
+/// This is important because we want the positions of introspectible elements
+/// that technically started before a pagebreak, but have no visible content
+/// yet, to be after the pagebreak. A typical case where this happens is `show
+/// heading: it => pagebreak() + it`.
+fn migrate_unterminated_tags(children: &mut [Pair], mid: usize) -> usize {
+    // Compute the range from before the first trailing tag to after the last
+    // following pagebreak.
+    let (before, after) = children.split_at(mid);
+    let start = mid - before.iter().rev().take_while(|&(c, _)| c.is::<TagElem>()).count();
+    let end = mid + after.iter().take_while(|&(c, _)| c.is::<PagebreakElem>()).count();
+
+    // Determine the set of tag locations which we won't migrate (because they
+    // are terminated).
+    let excluded: HashSet<_> = children[start..mid]
+        .iter()
+        .filter_map(|(c, _)| c.to_packed::<TagElem>())
+        .filter(|elem| elem.tag.kind() == TagKind::End)
+        .map(|elem| elem.tag.location())
+        .collect();
+
+    // A key function that partitions the area of interest into three groups:
+    // Excluded tags (-1) | Pagebreaks (0) | Migrated tags (1).
+    let key = |(c, _): &Pair| match c.to_packed::<TagElem>() {
+        Some(elem) => {
+            if excluded.contains(&elem.tag.location()) {
+                -1
+            } else {
+                1
+            }
+        }
+        None => 0,
+    };
+
+    // Partition the children using a *stable* sort. While it would be possible
+    // to write a more efficient direct algorithm for this, the sort version is
+    // less likely to have bugs and this is absolutely not on a hot path.
+    children[start..end].sort_by_key(key);
+
+    // Compute the new end index, right before the pagebreaks.
+    start + children[start..end].iter().take_while(|pair| key(pair) == -1).count()
+}
+
+/// Layout a page run with uniform properties.
+#[typst_macros::time(name = "page run")]
+fn layout_page_run(
+    engine: &mut Engine,
+    children: &[Pair],
+    locator: Locator,
+    initial: StyleChain,
+) -> SourceResult<Vec<LayoutedPage>> {
+    layout_page_run_impl(
+        engine.world,
+        engine.introspector,
+        engine.traced,
+        TrackedMut::reborrow_mut(&mut engine.sink),
+        engine.route.track(),
+        children,
+        locator.track(),
+        initial,
+    )
+}
+
+/// Layout a single page suitable  for parity adjustment.
+fn layout_blank_page(
+    engine: &mut Engine,
+    locator: Locator,
+    initial: StyleChain,
+) -> SourceResult<LayoutedPage> {
+    let layouted = layout_page_run(engine, &[], locator, initial)?;
+    Ok(layouted.into_iter().next().unwrap())
+}
+
+/// The internal implementation of `layout_page_run`.
+#[comemo::memoize]
+#[allow(clippy::too_many_arguments)]
+fn layout_page_run_impl(
+    world: Tracked<dyn World + '_>,
+    introspector: Tracked<Introspector>,
+    traced: Tracked<Traced>,
+    sink: TrackedMut<Sink>,
+    route: Tracked<Route>,
+    children: &[Pair],
+    locator: Tracked<Locator>,
+    initial: StyleChain,
+) -> SourceResult<Vec<LayoutedPage>> {
+    let link = LocatorLink::new(locator);
+    let mut locator = Locator::link(&link).split();
+    let mut engine = Engine {
+        world,
+        introspector,
+        traced,
+        sink,
+        route: Route::extend(route),
+    };
+
+    // Determine the page-wide styles.
+    let styles = determine_page_styles(children, initial);
+    let styles = StyleChain::new(&styles);
+
+    // When one of the lengths is infinite the page fits its content along
+    // that axis.
+    let width = PageElem::width_in(styles).unwrap_or(Abs::inf());
+    let height = PageElem::height_in(styles).unwrap_or(Abs::inf());
+    let mut size = Size::new(width, height);
+    if PageElem::flipped_in(styles) {
+        std::mem::swap(&mut size.x, &mut size.y);
+    }
+
+    let mut min = width.min(height);
+    if !min.is_finite() {
+        min = Paper::A4.width();
+    }
+
+    // Determine the margins.
+    let default = Rel::<Length>::from((2.5 / 21.0) * min);
+    let margin = PageElem::margin_in(styles);
+    let two_sided = margin.two_sided.unwrap_or(false);
+    let margin = margin
+        .sides
+        .map(|side| side.and_then(Smart::custom).unwrap_or(default))
+        .resolve(styles)
+        .relative_to(size);
+
+    // Realize columns.
+    let area = size - margin.sum_by_axis();
+    let mut regions = Regions::repeat(area, area.map(Abs::is_finite));
+    regions.root = true;
+
+    let fill = PageElem::fill_in(styles);
+    let foreground = PageElem::foreground_in(styles);
+    let background = PageElem::background_in(styles);
+    let header_ascent = PageElem::header_ascent_in(styles).relative_to(margin.top);
+    let footer_descent = PageElem::footer_descent_in(styles).relative_to(margin.bottom);
+    let numbering = PageElem::numbering_in(styles);
+    let number_align = PageElem::number_align_in(styles);
+    let binding =
+        PageElem::binding_in(styles).unwrap_or_else(|| match TextElem::dir_in(styles) {
+            Dir::LTR => Binding::Left,
+            _ => Binding::Right,
+        });
+
+    // Construct the numbering (for header or footer).
+    let numbering_marginal = numbering.as_ref().map(|numbering| {
+        let both = match numbering {
+            Numbering::Pattern(pattern) => pattern.pieces() >= 2,
+            Numbering::Func(_) => true,
+        };
+
+        let mut counter = CounterDisplayElem::new(
+            Counter::new(CounterKey::Page),
+            Smart::Custom(numbering.clone()),
+            both,
+        )
+        .pack();
+
+        // We interpret the Y alignment as selecting header or footer
+        // and then ignore it for aligning the actual number.
+        if let Some(x) = number_align.x() {
+            counter = counter.aligned(x.into());
+        }
+
+        counter
+    });
+
+    let header = PageElem::header_in(styles);
+    let footer = PageElem::footer_in(styles);
+    let (header, footer) = if matches!(number_align.y(), Some(OuterVAlignment::Top)) {
+        (header.as_ref().unwrap_or(&numbering_marginal), footer.as_ref().unwrap_or(&None))
+    } else {
+        (header.as_ref().unwrap_or(&None), footer.as_ref().unwrap_or(&numbering_marginal))
+    };
+
+    // Layout the children.
+    let bump = bumpalo::Bump::new();
+    let fragment = layout_flow(
+        &mut engine,
+        &bump,
+        children,
+        &mut locator,
+        styles,
+        regions,
+        PageElem::columns_in(styles),
+        ColumnsElem::gutter_in(styles),
+        Span::detached(),
+    )?;
+
+    // Layouts a single marginal.
+    let mut layout_marginal = |content: &Option<Content>, area, align| {
+        let Some(content) = content else { return Ok(None) };
+        let aligned = content.clone().styled(AlignElem::set_alignment(align));
+        layout_frame(
+            &mut engine,
+            &aligned,
+            locator.next(&content.span()),
+            styles,
+            Region::new(area, Axes::splat(true)),
+        )
+        .map(Some)
+    };
+
+    // Layout marginals.
+    let mut layouted = Vec::with_capacity(fragment.len());
+    for inner in fragment {
+        let header_size = Size::new(inner.width(), margin.top - header_ascent);
+        let footer_size = Size::new(inner.width(), margin.bottom - footer_descent);
+        let full_size = inner.size() + margin.sum_by_axis();
+        let mid = HAlignment::Center + VAlignment::Horizon;
+        layouted.push(LayoutedPage {
+            inner,
+            fill: fill.clone(),
+            numbering: numbering.clone(),
+            header: layout_marginal(header, header_size, Alignment::BOTTOM)?,
+            footer: layout_marginal(footer, footer_size, Alignment::TOP)?,
+            background: layout_marginal(background, full_size, mid)?,
+            foreground: layout_marginal(foreground, full_size, mid)?,
+            margin,
+            binding,
+            two_sided,
+        });
+    }
+
+    Ok(layouted)
+}
+
+/// Determines the styles used for a page run itself and page-level content like
+/// marginals and footnotes.
+///
+/// As a base, we collect the styles that are shared by all elements on the page
+/// run. As a fallback if there are no elements, we use the styles active at the
+/// pagebreak that introduced the page (at the very start, we use the default
+/// styles). Then, to produce our page styles, we filter this list of styles
+/// according to a few rules:
+///
+/// - Other styles are only kept if they are `outside && (initial || liftable)`.
+/// - "Outside" means they were not produced within a show rule or that the
+///   show rule "broke free" to the page level by emitting page styles.
+/// - "Initial" means they were active at the pagebreak that introduced the
+///   page. Since these are intuitively already active, they should be kept even
+///   if not liftable. (E.g. `text(red, page(..)`) makes the footer red.)
+/// - "Liftable" means they can be lifted to the page-level even though they
+///   weren't yet active at the very beginning. Set rule styles are liftable as
+///   opposed to direct constructor calls:
+///   - For `set page(..); set text(red)` the red text is kept even though it
+///     comes after the weak pagebreak from set page.
+///   - For `set page(..); text(red)[..]` the red isn't kept because the
+///     constructor styles are not liftable.
+fn determine_page_styles(children: &[Pair], initial: StyleChain) -> Styles {
+    // Determine the shared styles (excluding tags).
+    let tagless = children.iter().filter(|(c, _)| !c.is::<TagElem>()).map(|&(_, s)| s);
+    let base = StyleChain::trunk(tagless).unwrap_or(initial).to_map();
+
+    // Determine the initial styles that are also shared by everything. We can't
+    // use `StyleChain::trunk` because it currently doesn't deal with partially
+    // shared links (where a subslice matches).
+    let trunk_len = initial
+        .to_map()
+        .as_slice()
+        .iter()
+        .zip(base.as_slice())
+        .take_while(|&(a, b)| a == b)
+        .count();
+
+    // Filter the base styles according to our rules.
+    base.into_iter()
+        .enumerate()
+        .filter(|(i, style)| {
+            let initial = *i < trunk_len;
+            style.outside() && (initial || style.liftable())
+        })
+        .map(|(_, style)| style)
+        .collect()
+}
+
+/// Piece together the inner page frame and the marginals. We can only do this
+/// at the very end because inside/outside margins require knowledge of the
+/// physical page number, which is unknown during parallel layout.
+fn finalize_page(
+    engine: &mut Engine,
+    counter: &mut ManualPageCounter,
+    tags: &mut Vec<Tag>,
+    LayoutedPage {
+        inner,
+        mut margin,
+        binding,
+        two_sided,
+        header,
+        footer,
+        background,
+        foreground,
+        fill,
+        numbering,
+    }: LayoutedPage,
+) -> SourceResult<Page> {
+    // If two sided, left becomes inside and right becomes outside.
+    // Thus, for left-bound pages, we want to swap on even pages and
+    // for right-bound pages, we want to swap on odd pages.
+    if two_sided && binding.swap(counter.physical()) {
+        std::mem::swap(&mut margin.left, &mut margin.right);
+    }
+
+    // Create a frame for the full page.
+    let mut frame = Frame::hard(inner.size() + margin.sum_by_axis());
+
+    // Add tags.
+    for tag in tags.drain(..) {
+        frame.push(Point::zero(), FrameItem::Tag(tag));
+    }
+
+    // Add the "before" marginals. The order in which we push things here is
+    // important as it affects the relative ordering of introspectible elements
+    // and thus how counters resolve.
+    if let Some(background) = background {
+        frame.push_frame(Point::zero(), background);
+    }
+    if let Some(header) = header {
+        frame.push_frame(Point::with_x(margin.left), header);
+    }
+
+    // Add the inner contents.
+    frame.push_frame(Point::new(margin.left, margin.top), inner);
+
+    // Add the "after" marginals.
+    if let Some(footer) = footer {
+        let y = frame.height() - footer.height();
+        frame.push_frame(Point::new(margin.left, y), footer);
+    }
+    if let Some(foreground) = foreground {
+        frame.push_frame(Point::zero(), foreground);
+    }
+
+    // Apply counter updates from within the page to the manual page counter.
+    counter.visit(engine, &frame)?;
+
+    // Get this page's number and then bump the counter for the next page.
+    let number = counter.logical();
+    counter.step();
+
+    Ok(Page { frame, fill, numbering, number })
+}

crates/typst/src/layout/place.rs

@@ -1,10 +1,5 @@
-use crate::diag::{bail, At, Hint, SourceResult};
+use crate::foundations::{elem, scope, Content, Smart};
-use crate::engine::Engine;
+use crate::layout::{Alignment, Em, Length, Rel};
-use crate::foundations::{elem, scope, Content, Packed, Smart, StyleChain};
-use crate::introspection::Locator;
-use crate::layout::{
-    layout_frame, Alignment, Axes, Em, Frame, Length, Region, Rel, Size, VAlignment,
-};
 
 /// Places content at an absolute position.
 ///
@@ -103,42 +98,6 @@ impl PlaceElem {
     type FlushElem;
 }
 
-impl Packed<PlaceElem> {
-    #[typst_macros::time(name = "place", span = self.span())]
-    pub fn layout(
-        &self,
-        engine: &mut Engine,
-        locator: Locator,
-        styles: StyleChain,
-        base: Size,
-    ) -> SourceResult<Frame> {
-        // The pod is the base area of the region because for absolute
-        // placement we don't really care about the already used area.
-        let float = self.float(styles);
-        let alignment = self.alignment(styles);
-
-        if float
-            && alignment.is_custom_and(|align| {
-                matches!(align.y(), None | Some(VAlignment::Horizon))
-            })
-        {
-            bail!(self.span(), "floating placement must be `auto`, `top`, or `bottom`");
-        } else if !float && alignment.is_auto() {
-            return Err("automatic positioning is only available for floating placement")
-                .hint("you can enable floating placement with `place(float: true, ..)`")
-                .at(self.span());
-        }
-
-        let child = self
-            .body()
-            .clone()
-            .aligned(alignment.unwrap_or_else(|| Alignment::CENTER));
-
-        let pod = Region::new(base, Axes::splat(false));
-        layout_frame(engine, &child, locator, styles, pod)
-    }
-}
-
 /// Asks the layout algorithm to place pending floating elements before
 /// continuing with the content.
 ///

crates/typst/src/model/bibliography.rs

@@ -29,7 +29,7 @@ use crate::foundations::{
 };
 use crate::introspection::{Introspector, Locatable, Location};
 use crate::layout::{
-    BlockChild, BlockElem, Em, GridCell, GridChild, GridElem, GridItem, HElem, PadElem,
+    BlockBody, BlockElem, Em, GridCell, GridChild, GridElem, GridItem, HElem, PadElem,
     Sizing, TrackSizings, VElem,
 };
 use crate::model::{
@@ -932,7 +932,7 @@ impl ElemRenderer<'_> {
         match elem.display {
             Some(Display::Block) => {
                 content = BlockElem::new()
-                    .with_body(Some(BlockChild::Content(content)))
+                    .with_body(Some(BlockBody::Content(content)))
                     .pack()
                     .spanned(self.span);
             }

crates/typst/src/model/figure.rs

@@ -14,7 +14,7 @@ use crate::introspection::{
     Count, Counter, CounterKey, CounterUpdate, Locatable, Location,
 };
 use crate::layout::{
-    AlignElem, Alignment, BlockChild, BlockElem, Em, HAlignment, Length, OuterVAlignment,
+    AlignElem, Alignment, BlockBody, BlockElem, Em, HAlignment, Length, OuterVAlignment,
     PlaceElem, VAlignment, VElem,
 };
 use crate::model::{Numbering, NumberingPattern, Outlinable, Refable, Supplement};
@@ -318,7 +318,7 @@ impl Show for Packed<FigureElem> {
 
         // Wrap the contents in a block.
         realized = BlockElem::new()
-            .with_body(Some(BlockChild::Content(realized)))
+            .with_body(Some(BlockBody::Content(realized)))
             .pack()
             .spanned(self.span());
 

crates/typst/src/model/heading.rs

@@ -10,7 +10,7 @@ use crate::introspection::{
     Count, Counter, CounterUpdate, Locatable, Locator, LocatorLink,
 };
 use crate::layout::{
-    layout_frame, Abs, Axes, BlockChild, BlockElem, Em, HElem, Length, Region,
+    layout_frame, Abs, Axes, BlockBody, BlockElem, Em, HElem, Length, Region,
 };
 use crate::model::{Numbering, Outlinable, ParElem, Refable, Supplement};
 use crate::text::{FontWeight, LocalName, SpaceElem, TextElem, TextSize};
@@ -257,7 +257,7 @@ impl Show for Packed<HeadingElem> {
         }
 
         Ok(BlockElem::new()
-            .with_body(Some(BlockChild::Content(realized)))
+            .with_body(Some(BlockBody::Content(realized)))
             .pack()
             .spanned(span))
     }

crates/typst/src/model/quote.rs

@@ -6,7 +6,7 @@ use crate::foundations::{
 };
 use crate::introspection::Locatable;
 use crate::layout::{
-    Alignment, BlockChild, BlockElem, Em, HElem, PadElem, Spacing, VElem,
+    Alignment, BlockBody, BlockElem, Em, HElem, PadElem, Spacing, VElem,
 };
 use crate::model::{CitationForm, CiteElem};
 use crate::text::{SmartQuoteElem, SmartQuotes, SpaceElem, TextElem};
@@ -185,7 +185,7 @@ impl Show for Packed<QuoteElem> {
 
         if block {
             realized = BlockElem::new()
-                .with_body(Some(BlockChild::Content(realized)))
+                .with_body(Some(BlockBody::Content(realized)))
                 .pack()
                 .spanned(self.span());
 

crates/typst/src/realize.rs

@@ -82,11 +82,11 @@ pub enum RealizationKind<'a> {
 #[derive(Default)]
 pub struct Arenas {
     /// A typed arena for owned content.
-    content: typed_arena::Arena<Content>,
+    pub content: typed_arena::Arena<Content>,
     /// A typed arena for owned styles.
-    styles: typed_arena::Arena<Styles>,
+    pub styles: typed_arena::Arena<Styles>,
     /// An untyped arena for everything that is `Copy`.
-    bump: bumpalo::Bump,
+    pub bump: bumpalo::Bump,
 }
 
 /// Mutable state for realization.

crates/typst/src/text/raw.rs

@@ -16,7 +16,7 @@ use crate::foundations::{
     cast, elem, scope, Args, Array, Bytes, Content, Fold, NativeElement, Packed,
     PlainText, Show, ShowSet, Smart, StyleChain, Styles, Synthesize, Value,
 };
-use crate::layout::{BlockChild, BlockElem, Em, HAlignment};
+use crate::layout::{BlockBody, BlockElem, Em, HAlignment};
 use crate::model::{Figurable, ParElem};
 use crate::syntax::{split_newlines, LinkedNode, Span, Spanned};
 use crate::text::{
@@ -450,7 +450,7 @@ impl Show for Packed<RawElem> {
             // Align the text before inserting it into the block.
             realized = realized.aligned(self.align(styles).into());
             realized = BlockElem::new()
-                .with_body(Some(BlockChild::Content(realized)))
+                .with_body(Some(BlockBody::Content(realized)))
                 .pack()
                 .spanned(self.span());
         }

tests/suite/layout/pagebreak.typ

@@ -47,6 +47,7 @@ First
 --- pagebreak-in-container ---
 #box[
   // Error: 4-15 pagebreaks are not allowed inside of containers
+  // Hint: 4-15 try using a `#colbreak()` instead
   #pagebreak()
 ]