begin placing new headers

Considerations:
- Need to change layout headers algorithm to
  1. Place those headers
  2. But in a new region, also place other repeating headers
  3. Keep footer height up-to-date without much intervention

crates/typst-layout/src/grid/layouter.rs

@@ -55,11 +55,9 @@ pub struct GridLayouter<'a> {
     /// Note that some levels may be absent, in particular level 0, which does
     /// not exist (so the first level is >= 1).
     pub(super) repeating_headers: Vec<&'a Header>,
-    /// End of sequence of consecutive compatible headers found so far.
-    /// This is one position after the last index in `upcoming_headers`, so `0`
-    /// indicates no pending headers.
+    /// Headers, repeating or not, awaiting their first successful layout.
     /// Sorted by increasing levels.
-    pub(super) pending_header_end: usize,
+    pub(super) pending_headers: &'a [Repeatable<Header>],
     pub(super) upcoming_headers: &'a [Repeatable<Header>],
     /// The simulated header height.
     /// This field is reset in `layout_header` and properly updated by
@@ -136,7 +134,7 @@ impl<'a> GridLayouter<'a> {
             is_rtl: TextElem::dir_in(styles) == Dir::RTL,
             repeating_headers: vec![],
             upcoming_headers: &grid.headers,
-            pending_header_end: 0,
+            pending_headers: Default::default(),
             header_height: Abs::zero(),
             footer_height: Abs::zero(),
             span,
@@ -151,31 +149,34 @@ impl<'a> GridLayouter<'a> {
             // Ensure rows in the first region will be aware of the possible
             // presence of the footer.
             self.prepare_footer(footer, engine, 0)?;
-            if !matches!(
-                self.grid.headers.first(),
-                Some(Repeatable::Repeated(Header { start: 0, .. }))
-            ) {
-                // No repeatable header at the very beginning, so we won't
-                // subtract it later.
-                self.regions.size.y -= self.footer_height;
-            }
+            self.regions.size.y -= self.footer_height;
         }
 
         let mut y = 0;
+        let mut consecutive_header_count = 0;
         while y < self.grid.rows.len() {
-            if let Some(first_header) = self.upcoming_headers.first() {
+            if let Some(first_header) =
+                self.upcoming_headers.get(consecutive_header_count)
+            {
                 if first_header.unwrap().range().contains(&y) {
-                    self.bump_pending_headers();
+                    consecutive_header_count += 1;
 
-                    if self.peek_upcoming_header().is_none_or(|h| {
-                        h.unwrap().start > y + 1
-                            || h.unwrap().level <= first_header.unwrap().level
-                    }) {
+                    if self.upcoming_headers.get(consecutive_header_count).is_none_or(
+                        |h| {
+                            h.unwrap().start > y + 1
+                                || h.unwrap().level <= first_header.unwrap().level
+                        },
+                    ) {
                         // Next row either isn't a header. or is in a
                         // conflicting one, which is the sign that we need to go.
-                        self.layout_headers(next_header, engine, 0)?;
+                        self.place_new_headers(
+                            first_header,
+                            consecutive_header_count,
+                            engine,
+                        );
+                        consecutive_header_count = 0;
                     }
-                    y = first_header.end;
+                    y = first_header.unwrap().end;
                     // Skip header rows during normal layout.
                     continue;
 

crates/typst-layout/src/grid/repeated.rs

@@ -1,4 +1,3 @@
-use std::ops::ControlFlow;
 
 use typst_library::diag::SourceResult;
 use typst_library::engine::Engine;
@@ -9,20 +8,74 @@ use super::layouter::GridLayouter;
 use super::rowspans::UnbreakableRowGroup;
 
 impl<'a> GridLayouter<'a> {
-    #[inline]
-    fn pending_headers(&self) -> &'a [Repeatable<Header>] {
-        &self.upcoming_headers[..self.pending_header_end]
+    pub fn place_new_headers(
+        &mut self,
+        first_header: &Repeatable<Header>,
+        consecutive_header_count: usize,
+        engine: &mut Engine,
+    ) {
+        // Next row either isn't a header. or is in a
+        // conflicting one, which is the sign that we need to go.
+        let (consecutive_headers, new_upcoming_headers) =
+            self.upcoming_headers.split_at(consecutive_header_count);
+        self.upcoming_headers = new_upcoming_headers;
+
+        let (non_conflicting_headers, conflicting_headers) = match self
+            .upcoming_headers
+            .get(consecutive_header_count)
+            .map(Repeatable::unwrap)
+        {
+            Some(next_header) if next_header.level <= first_header.unwrap().level => {
+                // All immediately conflicting headers will
+                // be placed as normal rows.
+                consecutive_headers.split_at(
+                    consecutive_headers
+                        .partition_point(|h| next_header.level > h.unwrap().level),
+                )
+            }
+            _ => (consecutive_headers, Default::default()),
+        };
+
+        self.layout_new_pending_headers(non_conflicting_headers, engine);
+
+        self.layout_headers(non_conflicting_headers, engine, 0)?;
+        for conflicting_header in conflicting_headers {
+            self.simulate();
+            self.layout_headers(headers, engine, disambiguator)
+        }
     }
 
-    #[inline]
-    pub fn bump_pending_headers(&mut self) {
-        debug_assert!(!self.upcoming_headers.is_empty());
-        self.pending_header_end += 1;
-    }
+    /// Queues new pending headers for layout. Headers remain pending until
+    /// they are successfully laid out in some page once. Then, they will be
+    /// moved to `repeating_headers`, at which point it is safe to stop them
+    /// from repeating at any time.
+    fn layout_new_pending_headers(
+        &mut self,
+        headers: &'a [Repeatable<Header>],
+        engine: &mut Engine,
+    ) {
+        let [first_header, ..] = headers else {
+            return;
+        };
+        // Assuming non-conflicting headers sorted by increasing y, this must
+        // be the header with the lowest level (sorted by increasing levels).
+        let first_level = first_header.unwrap().level;
 
-    #[inline]
-    pub fn peek_upcoming_header(&self) -> Option<&'a Repeatable<Header>> {
-        self.upcoming_headers.get(self.pending_header_end)
+        // Stop repeating conflicting headers.
+        // If we go to a new region before the pending headers fit alongside
+        // their children, the old headers should not be displayed anymore.
+        self.repeating_headers
+            .truncate(self.repeating_headers.partition_point(|h| h.level < first_level));
+
+        // Let's try to place them at least once.
+        // This might be a waste as we could generate an orphan and thus have
+        // to try to place old and new headers all over again, but that happens
+        // for every new region anyway, so it's rather unavoidable.
+        self.layout_headers(headers.iter().map(Repeatable::unwrap), true, engine);
+
+        // After the first subsequent row is laid out, move to repeating, as
+        // it's then confirmed the headers won't be moved due to orphan
+        // prevention anymore.
     }
 
     pub fn flush_pending_headers(&mut self) {