Fix Behaviour of spacing (#3634)

crates/typst/src/introspection/metadata.rs

@@ -37,6 +37,8 @@ impl Show for Packed<MetadataElem> {
     }
 }
 
+// TODO: Metadata shows as empty content, so this behaviour is unnecessary. It
+// can be removed once 0.11 has landed.
 impl Behave for Packed<MetadataElem> {
     fn behaviour(&self) -> Behaviour {
         Behaviour::Invisible

crates/typst/src/layout/spacing.rs

@@ -68,7 +68,7 @@ impl Behave for Packed<HElem> {
         } else if self.weak(StyleChain::default()) {
             Behaviour::Weak(1)
         } else {
-            Behaviour::Invisible
+            Behaviour::Ignorant
         }
     }
 
@@ -166,7 +166,7 @@ impl Behave for Packed<VElem> {
         } else if self.weakness(StyleChain::default()) > 0 {
             Behaviour::Weak(self.weakness(StyleChain::default()))
         } else {
-            Behaviour::Invisible
+            Behaviour::Ignorant
         }
     }
 

crates/typst/src/realize/behaviour.rs

@@ -16,9 +16,11 @@ 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, but has a visual representation.
+    /// same effect as if it didn't exist, but has layout extent and/or a visual
+    /// representation.
     Ignorant,
-    /// An element that does not have a visual representation.
+    /// An element that does not have any layout extent or visual
+    /// representation.
     Invisible,
 }
 

crates/typst/src/realize/mod.rs

@@ -416,6 +416,9 @@ struct ParBuilder<'a>(BehavedBuilder<'a>);
 impl<'a> ParBuilder<'a> {
     fn accept(&mut self, content: &'a Content, styles: StyleChain<'a>) -> bool {
         if content.is::<MetaElem>() {
+            // TODO: This could probably just be `self.0.is_empty()` since no
+            // weak or invisible elements can be at the start of the builder. It
+            // can be removed once 0.11 has landed.
             if self.0.has_strong_elements(false) {
                 self.0.push(content, styles);
                 return true;

tests/typ/bugs/spacing-behaviour.typ

@@ -0,0 +1,9 @@
+// Test that metadata after spacing does not force a new paragraph.
+
+---
+#{
+  h(1em)
+  counter(heading).update(4)
+  [Hello ]
+  counter(heading).display()
+}