Introduce second variant of Ignorant Behaviour and prevent weak page breaks at basically empty pages (#1929)

crates/typst-library/src/layout/mod.rs

@@ -467,7 +467,7 @@ impl<'a, 'v, 't> Builder<'a, 'v, 't> {
     fn interrupt_page(&mut self, styles: Option<StyleChain<'a>>) -> SourceResult<()> {
         self.interrupt_par()?;
         let Some(doc) = &mut self.doc else { return Ok(()) };
-        if !self.flow.0.is_empty() || (doc.keep_next && styles.is_some()) {
+        if !self.flow.0.is_basically_empty() || (doc.keep_next && styles.is_some()) {
             let (flow, shared) = mem::take(&mut self.flow).0.finish();
             let styles = if shared == StyleChain::default() {
                 styles.unwrap_or_default()

crates/typst-library/src/layout/spacing.rs

@@ -63,7 +63,7 @@ impl Behave for HElem {
         } else if self.weak(StyleChain::default()) {
             Behaviour::Weak(1)
         } else {
-            Behaviour::Ignorant
+            Behaviour::Invisible
         }
     }
 
@@ -158,7 +158,7 @@ impl Behave for VElem {
         } else if self.weakness(StyleChain::default()) > 0 {
             Behaviour::Weak(self.weakness(StyleChain::default()))
         } else {
-            Behaviour::Ignorant
+            Behaviour::Invisible
         }
     }
 

crates/typst-library/src/meta/metadata.rs

@@ -38,6 +38,6 @@ impl Show for MetadataElem {
 
 impl Behave for MetadataElem {
     fn behaviour(&self) -> Behaviour {
-        Behaviour::Ignorant
+        Behaviour::Invisible
     }
 }

crates/typst-library/src/shared/behave.rs

@@ -34,10 +34,9 @@ impl<'a> BehavedBuilder<'a> {
     /// probably collapse.
     pub fn is_basically_empty(&self) -> bool {
         self.builder.is_empty()
-            && self
+            && self.staged.iter().all(|(_, behaviour, _)| {
-                .staged
+                matches!(behaviour, Behaviour::Weak(_) | Behaviour::Invisible)
-                .iter()
+            })
-                .all(|(_, behaviour, _)| matches!(behaviour, Behaviour::Weak(_)))
     }
 
     /// Push an item into the sequence.
@@ -74,7 +73,7 @@ impl<'a> BehavedBuilder<'a> {
                 self.builder.push(elem, styles);
                 self.last = interaction;
             }
-            Behaviour::Ignorant => {
+            Behaviour::Ignorant | Behaviour::Invisible => {
                 self.staged.push((elem, interaction, styles));
             }
         }
@@ -95,7 +94,10 @@ impl<'a> BehavedBuilder<'a> {
     /// false.
     fn flush(&mut self, supportive: bool) {
         for (item, interaction, styles) in self.staged.drain(..) {
-            if supportive || interaction == Behaviour::Ignorant {
+            if supportive
+                || interaction == Behaviour::Ignorant
+                || interaction == Behaviour::Invisible
+            {
                 self.builder.push(item, styles);
             }
         }

crates/typst/src/model/content.rs

@@ -610,7 +610,7 @@ pub struct MetaElem {
 
 impl Behave for MetaElem {
     fn behaviour(&self) -> Behaviour {
-        Behaviour::Ignorant
+        Behaviour::Invisible
     }
 }
 

crates/typst/src/model/realize.rs

@@ -214,8 +214,10 @@ pub enum Behaviour {
     /// An element that destroys adjacent weak elements.
     Destructive,
     /// An element that does not interact at all with other elements, having the
-    /// same effect as if it didn't exist.
+    /// same effect as if it didn't exist, but has a visual representation.
     Ignorant,
+    /// An element that does not have a visual representation.
+    Invisible,
 }
 
 /// Guards content against being affected by the same show rule multiple times.

tests/typ/layout/pagebreak-empty.typ

@@ -0,0 +1,30 @@
+// Test page breaks on basically empty pages.
+
+---
+// After place
+// Should result in three pages.
+First
+#pagebreak(weak: true)
+#place(right)[placed A]
+#pagebreak(weak: true)
+Third
+
+---
+// After only ignorables & invisibles
+// Should result in two pages.
+First
+#pagebreak(weak: true)
+#counter(page).update(1)
+#metadata("Some")
+#pagebreak(weak: true)
+Second
+
+---
+// After only ignorables, but regular break
+// Should result in three pages.
+First
+#pagebreak()
+#counter(page).update(1)
+#metadata("Some")
+#pagebreak()
+Third