allow skipping from non-repeated header on lack of space

Can lead to orphans, but without enough space, anything goes.

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

@@ -1376,14 +1376,13 @@ impl<'a> GridLayouter<'a> {
         // endlessly repeated) when subtracting header and footer height.
         //
         // See 'check_for_unbreakable_rows' as for why we're using
-        // 'header_height' to predict header height and not
-        // 'repeating_header_height'.
+        // 'repeating_header_height' to predict header height.
         let height = frame.height();
         while self.unbreakable_rows_left == 0
             && !self.regions.size.y.fits(height)
             && may_progress_with_offset(
                 self.regions,
-                self.current.header_height + self.current.footer_height,
+                self.current.repeating_header_height + self.current.footer_height,
             )
         {
             self.finish_region(engine, false)?;
@@ -1571,11 +1570,9 @@ impl<'a> GridLayouter<'a> {
                 && self.current.lrows.is_empty()
                 && may_progress_with_offset(
                     self.regions,
-                    // This header height isn't doing much as we just
-                    // confirmed that there are no headers in this region,
-                    // but let's keep it here for correctness. It will add
-                    // zero anyway.
-                    self.current.header_height + self.current.footer_height,
+                    // Don't sum header height as we just confirmed that there
+                    // are no headers in this region.
+                    self.current.footer_height,
                 );
 
         let mut laid_out_footer_start = None;
@@ -1802,6 +1799,9 @@ pub(super) fn points(
 /// additional logic which adds content automatically on each region turn (in
 /// our case, headers).
 pub(super) fn may_progress_with_offset(regions: Regions<'_>, offset: Abs) -> bool {
+    // Use 'approx_eq' as float addition and subtraction are not associative.
     !regions.backlog.is_empty()
-        || regions.last.is_some_and(|height| regions.size.y + offset != height)
+        || regions
+            .last
+            .is_some_and(|height| !(regions.size.y + offset).approx_eq(height))
 }

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

@@ -377,7 +377,7 @@ impl<'a> GridLayouter<'a> {
                 // 'header_height == repeating_header_height' here
                 // (there won't be any pending headers at this point, other
                 // than the ones we are about to place).
-                self.current.header_height + self.current.footer_height,
+                self.current.repeating_header_height + self.current.footer_height,
             )
         {
             // Note that, after the first region skip, the new headers will go
@@ -393,7 +393,7 @@ impl<'a> GridLayouter<'a> {
             && self.current.lrows_orphan_snapshot.is_none()
             && may_progress_with_offset(
                 self.regions,
-                self.current.header_height + self.current.footer_height,
+                self.current.repeating_header_height + self.current.footer_height,
             );
 
         if should_snapshot {

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

@@ -258,12 +258,11 @@ impl GridLayouter<'_> {
             while !self.regions.size.y.fits(row_group.height)
                 && may_progress_with_offset(
                     self.regions,
-                    // Note that we consider that the exact same headers and footers will be
-                    // added if we skip like this (blocking other rows from being laid out)
-                    // due to orphan/widow prevention, which explains the usage of
-                    // 'header_height' (include non-repeating but pending headers) rather
-                    // than 'repeating_header_height'.
-                    self.current.header_height + self.current.footer_height,
+                    // Use 'repeating_header_height' (ignoring the height of
+                    // non-repeated headers) to allow skipping if the
+                    // non-repeated header is too large. It will become an
+                    // orphan, but when there is no space left, anything goes.
+                    self.current.repeating_header_height + self.current.footer_height,
                 )
             {
                 self.finish_region(engine, false)?;

tests/suite/layout/grid/subheaders.typ

@@ -815,3 +815,57 @@
     [a],
   )
 )
+
+--- grid-subheaders-too-large-non-repeating-orphan-before-auto ---
+#set page(height: 8em)
+#grid(
+  grid.header([1]),
+  grid.header(
+    [a\ ] * 2,
+    level: 2,
+    repeat: false,
+  ),
+  grid.header([2], level: 3),
+  [b\ b\ b],
+)
+
+--- grid-subheaders-too-large-repeating-orphan-before-auto ---
+#set page(height: 8em)
+#grid(
+  grid.header([1]),
+  grid.header(
+    [a\ ] * 2,
+    level: 2,
+    repeat: true,
+  ),
+  grid.header([2], level: 3),
+  rect(width: 10pt, height: 3em, fill: red),
+)
+
+--- grid-subheaders-too-large-repeating-orphan-before-relative ---
+#set page(height: 8em)
+#grid(
+  rows: (auto, auto, auto, 3em),
+  grid.header([1]),
+  grid.header(
+    [a\ ] * 2,
+    level: 2,
+    repeat: true,
+  ),
+  grid.header([2], level: 3),
+  rect(width: 10pt, height: 3em, fill: red),
+)
+
+--- grid-subheaders-too-large-non-repeating-orphan-before-relative ---
+#set page(height: 8em)
+#grid(
+  rows: (auto, auto, auto, 3em),
+  grid.header([1]),
+  grid.header(
+    [a\ ] * 2,
+    level: 2,
+    repeat: false,
+  ),
+  grid.header([2], level: 3),
+  rect(width: 10pt, height: 3em, fill: red),
+)