Allow sticky blocks to be breakable (#5161)

crates/typst/src/layout/container.rs

@@ -422,8 +422,6 @@ pub struct BlockElem {
     /// This is, by default, set on heading blocks to prevent orphaned headings
     /// at the bottom of the page.
     ///
-    /// Marking a block as sticky makes it unbreakable.
-    ///
     /// ```example
     /// >>> #set page(height: 140pt)
     /// // Disable stickiness of headings.

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

@@ -189,7 +189,7 @@ impl<'a> Collector<'a, '_, '_> {
 
         self.output.push(spacing(elem.above(styles)));
 
-        if !breakable || sticky || fr.is_some() {
+        if !breakable || fr.is_some() {
             self.output.push(Child::Single(self.boxed(SingleChild {
                 align,
                 sticky,
@@ -203,6 +203,7 @@ impl<'a> Collector<'a, '_, '_> {
             let alone = self.children.len() == 1;
             self.output.push(Child::Multi(self.boxed(MultiChild {
                 align,
+                sticky,
                 alone,
                 elem,
                 styles,
@@ -375,6 +376,7 @@ fn layout_single_impl(
 #[derive(Debug)]
 pub struct MultiChild<'a> {
     pub align: Axes<FixedAlignment>,
+    pub sticky: bool,
     alone: bool,
     elem: &'a Packed<BlockElem>,
     styles: StyleChain<'a>,

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

@@ -247,7 +247,7 @@ impl<'a, 'b> Distributor<'a, 'b, '_, '_, '_> {
 
         // Lay out the block.
         let (frame, spill) = multi.layout(self.composer.engine, self.regions)?;
-        self.frame(frame, multi.align, false, true)?;
+        self.frame(frame, multi.align, multi.sticky, true)?;
 
         // If the block didn't fully fit into the current region, save it into
         // the `spill` and finish the region.
@@ -292,9 +292,9 @@ impl<'a, 'b> Distributor<'a, 'b, '_, '_, '_> {
         breakable: bool,
     ) -> FlowResult<()> {
         if sticky {
-            // If the frame is sticky and we haven't remember a preceding sticky
-            // element, make a checkpoint which we can restore should we end on
-            // this sticky element.
+            // If the frame is sticky and we haven't remembered a preceding
+            // sticky element, make a checkpoint which we can restore should we
+            // end on this sticky element.
             if self.stickable && self.sticky.is_none() {
                 self.sticky = Some(self.snapshot());
             }

tests/suite/layout/container.typ

@@ -180,6 +180,12 @@ A
 #colbreak()
 C
 
+--- block-sticky-breakable ---
+// Ensure that sticky blocks are still breakable.
+#set page(height: 60pt)
+#block(sticky: true, lines(4))
+E
+
 --- box-clip-rect ---
 // Test box clipping with a rectangle
 Hello #box(width: 1em, height: 1em, clip: false)[#rect(width: 3em, height: 3em, fill: red)]