Improve region progression handling

Fixes #5044

crates/typst/src/layout/container.rs

@@ -925,18 +925,22 @@ fn breakable_pod<'a>(
 /// height and a new backlog.
 fn distribute<'a>(
     height: Abs,
-    regions: Regions,
+    mut regions: Regions,
     buf: &'a mut SmallVec<[Abs; 2]>,
 ) -> (Abs, &'a mut [Abs]) {
     // Build new region heights from old regions.
     let mut remaining = height;
-    for region in regions.iter() {
-        let limited = region.y.min(remaining);
+    loop {
+        let limited = regions.size.y.clamp(Abs::zero(), remaining);
         buf.push(limited);
         remaining -= limited;
-        if remaining.approx_empty() {
+        if remaining.approx_empty()
+            || !regions.may_break()
+            || (!regions.may_progress() && limited.approx_empty())
+        {
             break;
         }
+        regions.next();
     }
 
     // If there is still something remaining, apply it to the

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

@@ -282,7 +282,7 @@ impl<'a, 'b> Composer<'a, 'b, '_, '_> {
         let need = frame.height() + clearance;
 
         // If the float doesn't fit, queue it for the next region.
-        if !remaining.fits(need) && !regions.in_last() {
+        if !remaining.fits(need) && regions.may_progress() {
             self.work.floats.push(placed);
             return Ok(());
         }
@@ -343,7 +343,7 @@ impl<'a, 'b> Composer<'a, 'b, '_, '_> {
 
         let mut relayout = false;
         let mut regions = *regions;
-        let mut migratable = !breakable && !regions.in_last();
+        let mut migratable = !breakable && regions.may_progress();
 
         for (y, elem) in notes {
             // The amount of space used by the in-flow content that contains the

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

@@ -194,9 +194,9 @@ impl<'a, 'b> Distributor<'a, 'b, '_, '_, '_> {
 
     /// Processes a line of a paragraph.
     fn line(&mut self, line: &'b LineChild) -> FlowResult<()> {
-        // If the line doesn't fit and we're allowed to break, finish the
-        // region.
-        if !self.regions.size.y.fits(line.frame.height()) && !self.regions.in_last() {
+        // If the line doesn't fit and a followup region may improve things,
+        // finish the region.
+        if !self.regions.size.y.fits(line.frame.height()) && self.regions.may_progress() {
             return Err(Stop::Finish(false));
         }
 
@@ -228,9 +228,9 @@ impl<'a, 'b> Distributor<'a, 'b, '_, '_, '_> {
         // Lay out the block.
         let frame = single.layout(self.composer.engine, self.regions.base())?;
 
-        // If the block doesn't fit and we're allowed to break, finish the
-        // region.
-        if !self.regions.size.y.fits(frame.height()) && !self.regions.in_last() {
+        // If the block doesn't fit and a followup region may improve things,
+        // finish the region.
+        if !self.regions.size.y.fits(frame.height()) && self.regions.may_progress() {
             return Err(Stop::Finish(false));
         }
 

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

@@ -57,7 +57,7 @@ impl<'a> GridLayouter<'a> {
         let mut skipped_region = false;
         while self.unbreakable_rows_left == 0
             && !self.regions.size.y.fits(header_rows.height + self.footer_height)
-            && !self.regions.in_last()
+            && self.regions.may_progress()
         {
             // Advance regions without any output until we can place the
             // header and the footer.
@@ -124,7 +124,7 @@ impl<'a> GridLayouter<'a> {
         let mut skipped_region = false;
         while self.unbreakable_rows_left == 0
             && !self.regions.size.y.fits(footer_height)
-            && !self.regions.in_last()
+            && self.regions.may_progress()
         {
             // Advance regions without any output until we can place the
             // footer.

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

@@ -1147,7 +1147,7 @@ impl<'a> RowspanSimulator<'a> {
 
         // Skip until we reach a fitting region for both header and footer.
         while !self.regions.size.y.fits(header_height + footer_height)
-            && !self.regions.in_last()
+            && self.regions.may_progress()
         {
             self.regions.next();
             self.finished += 1;

crates/typst/src/layout/regions.rs

@@ -96,14 +96,18 @@ impl Regions<'_> {
 
     /// Whether the first region is full and a region break is called for.
     pub fn is_full(&self) -> bool {
-        Abs::zero().fits(self.size.y) && !self.in_last()
+        Abs::zero().fits(self.size.y) && self.may_progress()
     }
 
-    /// Whether the first region is the last usable region.
-    ///
-    /// If this is true, calling `next()` will have no effect.
-    pub fn in_last(&self) -> bool {
-        self.backlog.is_empty() && self.last.map_or(true, |height| self.size.y == height)
+    /// Whether a region break is permitted.
+    pub fn may_break(&self) -> bool {
+        !self.backlog.is_empty() || self.last.is_some()
+    }
+
+    /// Whether calling `next()` may improve a situation where there is a lack
+    /// of space.
+    pub fn may_progress(&self) -> bool {
+        !self.backlog.is_empty() || self.last.is_some_and(|height| self.size.y != height)
     }
 
     /// Advance to the next region if there is any.

crates/typst/src/math/equation.rs

@@ -327,8 +327,9 @@ fn layout_equation_block(
         let mut rows = full_equation_builder.frames.into_iter().peekable();
         let mut equation_builders = vec![];
         let mut last_first_pos = Point::zero();
+        let mut regions = regions;
 
-        for region in regions.iter() {
+        loop {
             // Keep track of the position of the first row in this region,
             // so that the offset can be reverted later.
             let Some(&(_, first_pos)) = rows.peek() else { break };
@@ -344,8 +345,9 @@ fn layout_equation_block(
                 // we placed at least one line _or_ we still have non-last
                 // regions. Crucially, we don't want to infinitely create
                 // new regions which are too small.
-                if !region.y.fits(sub.height() + pos.y)
-                    && (!frames.is_empty() || !regions.in_last())
+                if !regions.size.y.fits(sub.height() + pos.y)
+                    && (regions.may_progress()
+                        || (regions.may_break() && !frames.is_empty()))
                 {
                     break;
                 }
@@ -357,6 +359,7 @@ fn layout_equation_block(
 
             equation_builders
                 .push(MathRunFrameBuilder { frames, size: Size::new(width, height) });
+            regions.next();
         }
 
         // Append remaining rows to the equation builder of the last region.

tests/suite/layout/pad.typ

@@ -28,3 +28,7 @@ Hi #box(pad(left: 10pt)[A]) there
 --- pad-adding-to-100-percent ---
 // Test that padding adding up to 100% does not panic.
 #pad(50%)[]
+
+--- issue-5044-pad-100-percent ---
+#set page(width: 30pt, height: 30pt)
+#pad(100%, block(width: 1cm, height: 1cm, fill: red))