Use line height based spacing model

crates/typst/src/layout/flow.rs

@@ -263,7 +263,7 @@ impl<'a> FlowLayouter<'a> {
         styles: StyleChain,
     ) -> SourceResult<()> {
         let align = AlignElem::alignment_in(styles).resolve(styles);
-        let leading = ParElem::leading_in(styles);
+        let line_height = ParElem::line_height_in(styles);
         let consecutive = self.last_was_par;
         let lines = par
             .layout(
@@ -303,8 +303,14 @@ impl<'a> FlowLayouter<'a> {
             }
         }
 
-        for (i, mut frame) in lines.into_iter().enumerate() {
+        for (i, mut frame) in lines.clone().into_iter().enumerate() {
             if i > 0 {
+                let leading =
+                    if lines[i - 1].descent().abs() + frame.ascent() > line_height {
+                        Abs::pt(1.0)
+                    } else {
+                        line_height - lines[i - 1].descent().abs() - frame.ascent()
+                    };
                 self.layout_item(engine, FlowItem::Absolute(leading, true))?;
             }
 

crates/typst/src/layout/inline/mod.rs

@@ -141,8 +141,8 @@ struct Preparation<'a> {
     cjk_latin_spacing: bool,
     /// Whether font fallback is enabled for this paragraph.
     fallback: bool,
-    /// The leading of the paragraph.
-    leading: Abs,
+    /// The line height of the paragraph.
+    line_height: Abs,
     /// How to determine line breaks.
     linebreaks: Smart<Linebreaks>,
     /// The text size.
@@ -682,7 +682,7 @@ fn prepare<'a>(
         hang: ParElem::hanging_indent_in(styles),
         cjk_latin_spacing,
         fallback: TextElem::fallback_in(styles),
-        leading: ParElem::leading_in(styles),
+        line_height: ParElem::line_height_in(styles),
         linebreaks: ParElem::linebreaks_in(styles),
         size: TextElem::size_in(styles),
     })
@@ -1292,7 +1292,7 @@ fn finalize(
         if frames.len() >= 2 && !frames[1].is_empty() {
             let second = frames.remove(1);
             let first = &mut frames[0];
-            merge(first, second, p.leading);
+            merge(first, second, p.line_height, true);
         }
     }
     if p.costs.widow().get() > 0.0 {
@@ -1301,19 +1301,28 @@ fn finalize(
         if len >= 2 && !frames[len - 2].is_empty() {
             let second = frames.pop().unwrap();
             let first = frames.last_mut().unwrap();
-            merge(first, second, p.leading);
+            merge(first, second, p.line_height, false);
         }
     }
 
     Ok(Fragment::frames(frames))
 }
 
 /// Merge two line frames
-fn merge(first: &mut Frame, second: Frame, leading: Abs) {
+fn merge(first: &mut Frame, second: Frame, line_height: Abs, based_on_second: bool) {
+    let leading = if first.descent().abs() + second.ascent().abs() > line_height {
+        Abs::pt(1.0)
+    } else {
+        line_height - first.descent().abs() - second.ascent().abs()
+    };
     let offset = first.height() + leading;
     let total = offset + second.height();
+    let second_baseline = offset + second.baseline();
     first.push_frame(Point::with_y(offset), second);
     first.size_mut().y = total;
+    if based_on_second {
+        first.set_baseline(second_baseline);
+    }
 }
 
 /// Commit to a line and build its frame.

crates/typst/src/math/equation.rs

@@ -17,7 +17,7 @@ use crate::layout::{
 use crate::math::{
     scaled_font_size, LayoutMath, MathContext, MathRunFrameBuilder, MathSize, MathVariant,
 };
-use crate::model::{Numbering, Outlinable, ParElem, Refable, Supplement};
+use crate::model::{Numbering, Outlinable, Refable, Supplement};
 use crate::syntax::Span;
 use crate::text::{
     families, variant, Font, FontFamily, FontList, FontWeight, LocalName, TextElem,
@@ -297,13 +297,8 @@ fn layout_equation_inline(
         let InlineItem::Frame(frame) = item else { continue };
 
         let font_size = scaled_font_size(&ctx, styles);
-        let slack = ParElem::leading_in(styles) * 0.7;
-        let top_edge = TextElem::top_edge_in(styles).resolve(font_size, &font, None);
-        let bottom_edge =
-            -TextElem::bottom_edge_in(styles).resolve(font_size, &font, None);
-
-        let ascent = top_edge.max(frame.ascent() - slack);
-        let descent = bottom_edge.max(frame.descent() - slack);
+        let ascent = TextElem::top_edge_in(styles).resolve(font_size, &font, None);
+        let descent = -TextElem::bottom_edge_in(styles).resolve(font_size, &font, None);
         frame.translate(Point::with_y(ascent - frame.baseline()));
         frame.size_mut().y = ascent + descent;
     }

crates/typst/src/math/row.rs

@@ -3,17 +3,12 @@ use std::iter::once;
 use unicode_math_class::MathClass;
 
 use crate::foundations::{Resolve, StyleChain};
-use crate::layout::{Abs, AlignElem, Em, Frame, InlineItem, Point, Size};
-use crate::math::{
-    alignments, scaled_font_size, spacing, EquationElem, FrameFragment, MathContext,
-    MathFragment, MathSize,
-};
+use crate::layout::{Abs, AlignElem, Frame, InlineItem, Point, Size};
+use crate::math::{alignments, spacing, FrameFragment, MathContext, MathFragment};
 use crate::model::ParElem;
 
 use super::fragment::SpacingFragment;
 
-pub const TIGHT_LEADING: Em = Em::new(0.25);
-
 /// A linear collection of [`MathFragment`]s.
 #[derive(Debug, Default, Clone)]
 pub struct MathRun(Vec<MathFragment>);
@@ -170,27 +165,31 @@ impl MathRun {
         ctx: &MathContext,
         styles: StyleChain,
     ) -> MathRunFrameBuilder {
+        let _ = ctx;
         let rows: Vec<_> = self.rows();
         let row_count = rows.len();
         let alignments = alignments(&rows);
 
-        let leading = if EquationElem::size_in(styles) >= MathSize::Text {
-            ParElem::leading_in(styles)
-        } else {
-            let font_size = scaled_font_size(ctx, styles);
-            TIGHT_LEADING.at(font_size)
-        };
+        let line_height = ParElem::line_height_in(styles);
 
         let align = AlignElem::alignment_in(styles).resolve(styles).x;
         let mut frames: Vec<(Frame, Point)> = vec![];
         let mut size = Size::zero();
-        for (i, row) in rows.into_iter().enumerate() {
+        for (i, row) in rows.clone().into_iter().enumerate() {
             if i == row_count - 1 && row.0.is_empty() {
                 continue;
             }
 
-            let sub = row.into_line_frame(&alignments.points, LeftRightAlternator::Right);
+            let sub = row
+                .clone()
+                .into_line_frame(&alignments.points, LeftRightAlternator::Right);
             if i > 0 {
+                let leading =
+                    if rows[i - 1].descent().abs() + row.ascent().abs() > line_height {
+                        Abs::pt(1.0)
+                    } else {
+                        line_height - rows[i - 1].descent().abs() - row.ascent().abs()
+                    };
                 size.y += leading;
             }
 

crates/typst/src/model/enum.rs

@@ -11,8 +11,8 @@ use crate::foundations::{
 };
 use crate::introspection::Locator;
 use crate::layout::{
-    Alignment, Axes, BlockElem, Cell, CellGrid, Em, Fragment, GridLayouter, HAlignment,
-    Length, Regions, Sizing, VAlignment, VElem,
+    Abs, Alignment, Axes, BlockElem, Cell, CellGrid, Em, Fragment, GridLayouter,
+    HAlignment, Length, Regions, Sizing, Spacing, VAlignment, VElem,
 };
 use crate::model::{Numbering, NumberingPattern, ParElem};
 use crate::text::TextElem;
@@ -223,7 +223,8 @@ impl Show for Packed<EnumElem> {
             .spanned(self.span());
 
         if self.tight(styles) {
-            let leading = ParElem::leading_in(styles);
+            // TODO(mgt): fix this
+            let leading: Abs = ParElem::line_height_in(styles);
             let spacing = VElem::list_attach(leading.into()).pack();
             realized = spacing + realized;
         }
@@ -246,7 +247,8 @@ fn layout_enum(
     let body_indent = elem.body_indent(styles);
     let gutter = elem.spacing(styles).unwrap_or_else(|| {
         if elem.tight(styles) {
-            ParElem::leading_in(styles).into()
+            // TODO(mgt): fix this
+            ParElem::line_height_in(styles).into()
         } else {
             ParElem::spacing_in(styles).into()
         }

crates/typst/src/model/footnote.rs

@@ -299,9 +299,9 @@ impl Show for Packed<FootnoteEntry> {
 impl ShowSet for Packed<FootnoteEntry> {
     fn show_set(&self, _: StyleChain) -> Styles {
         let text_size = Em::new(0.85);
-        let leading = Em::new(0.5);
+        let line_height = Em::new(1.158203125);
         let mut out = Styles::new();
-        out.set(ParElem::set_leading(leading.into()));
+        out.set(ParElem::set_line_height(line_height.into()));
         out.set(TextElem::set_size(TextSize(text_size.into())));
         out
     }

crates/typst/src/model/list.rs

@@ -147,7 +147,8 @@ impl Show for Packed<ListElem> {
             .spanned(self.span());
 
         if self.tight(styles) {
-            let leading = ParElem::leading_in(styles);
+            // TODO(mgt): fix this
+            let leading = ParElem::line_height_in(styles);
             let spacing = VElem::list_attach(leading.into()).pack();
             realized = spacing + realized;
         }
@@ -169,7 +170,8 @@ fn layout_list(
     let body_indent = elem.body_indent(styles);
     let gutter = elem.spacing(styles).unwrap_or_else(|| {
         if elem.tight(styles) {
-            ParElem::leading_in(styles).into()
+            // TODO(mgt): fix this
+            ParElem::line_height_in(styles).into()
         } else {
             ParElem::spacing_in(styles).into()
         }

crates/typst/src/model/par.rs

@@ -37,38 +37,11 @@ use crate::realize::StyleVec;
 /// ```
 #[elem(title = "Paragraph", Debug, Construct)]
 pub struct ParElem {
-    /// The spacing between lines.
-    ///
-    /// Leading defines the spacing between the [bottom edge]($text.bottom-edge)
-    /// of one line and the [top edge]($text.top-edge) of the following line. By
-    /// default, these two properties are up to the font, but they can also be
-    /// configured manually with a text set rule.
-    ///
-    /// By setting top edge, bottom edge, and leading, you can also configure a
-    /// consistent baseline-to-baseline distance. You could, for instance, set
-    /// the leading to `{1em}`, the top-edge to `{0.8em}`, and the bottom-edge
-    /// to `-{0.2em}` to get a baseline gap of exactly `{2em}`. The exact
-    /// distribution of the top- and bottom-edge values affects the bounds of
-    /// the first and last line.
-    #[resolve]
-    #[ghost]
-    #[default(Em::new(0.65).into())]
-    pub leading: Length,
-
-    /// The spacing between paragraphs.
-    ///
-    /// Just like leading, this defines the spacing between the bottom edge of a
-    /// paragraph's last line and the top edge of the next paragraph's first
-    /// line.
-    ///
-    /// When a paragraph is adjacent to a [`block`] that is not a paragraph,
-    /// that block's [`above`]($block.above) or [`below`]($block.below) property
-    /// takes precedence over the paragraph spacing. Headings, for instance,
-    /// reduce the spacing below them by default for a better look.
+    /// The spacing between baselines.
     #[resolve]
     #[ghost]
-    #[default(Em::new(1.2).into())]
-    pub spacing: Length,
+    #[default(Em::new(1.308203125).into())]
+    pub line_height: Length,
 
     /// Whether to justify text in its line.
     ///

crates/typst/src/model/terms.rs

@@ -116,7 +116,8 @@ impl Show for Packed<TermsElem> {
         let hanging_indent = self.hanging_indent(styles);
         let gutter = self.spacing(styles).unwrap_or_else(|| {
             if self.tight(styles) {
-                ParElem::leading_in(styles).into()
+                // TODO(mgt): fix this
+                ParElem::line_height_in(styles).into()
             } else {
                 ParElem::spacing_in(styles).into()
             }
@@ -149,7 +150,8 @@ impl Show for Packed<TermsElem> {
             .padded(padding);
 
         if self.tight(styles) {
-            let leading = ParElem::leading_in(styles);
+            // TODO(mgt): fix this
+            let leading = ParElem::line_height_in(styles);
             let spacing = VElem::list_attach(leading.into()).pack();
             realized = spacing + realized;
         }

tests/suite/model/par.typ

@@ -78,3 +78,10 @@ Welcome \ here. Does this work well?
 #set text(dir: rtl)
 لآن وقد أظلم الليل وبدأت النجوم
 تنضخ وجه الطبيعة التي أعْيَتْ من طول ما انبعثت في النهار
+
+--- par-inline-math-overlap ---
+#lorem(1) $1/123456$ #lorem(2) $12345667/1/1$
+
+#lorem(3) $sum_(x=0)^n$ #lorem(3)
+
+#lorem(3) $display((a+b)/(c+d))$ #lorem(2) $display((a/b+1)/c+1)$ #lorem(7)