Use CSS definition of line height (linebender#84)

Implements CSS style line height computation.

This is now directly on top of main and linebender#76 builds on top of this.

Author: nicoburns

parley/src/layout/line/greedy.rs

@@ -472,6 +472,7 @@ impl<'a, B: Brush> BreakLines<'a, B> {
                         // Default vertical alignment is to align the bottom of boxes with the text baseline.
                         // This is equivalent to the entire height of the box being "ascent"
                         line.metrics.ascent = line.metrics.ascent.max(item.height);
+                        line.metrics.line_height = line.metrics.line_height.max(item.height);
 
                         // Mark us as having seen non-whitespace content on this line
                         have_metrics = true;
@@ -504,12 +505,10 @@ impl<'a, B: Brush> BreakLines<'a, B> {
                         // Compute the run's vertical metrics
                         let run = &self.layout.runs[line_item.index];
                         let line_height = line_item.compute_line_height(self.layout);
-                        line.metrics.ascent =
-                            line.metrics.ascent.max(run.metrics.ascent * line_height);
-                        line.metrics.descent =
-                            line.metrics.descent.max(run.metrics.descent * line_height);
-                        line.metrics.leading =
-                            line.metrics.leading.max(run.metrics.leading * line_height);
+                        line.metrics.line_height = line.metrics.line_height.max(line_height);
+                        line.metrics.ascent = line.metrics.ascent.max(run.metrics.ascent);
+                        line.metrics.descent = line.metrics.descent.max(run.metrics.descent);
+                        line.metrics.leading = line.metrics.leading.max(run.metrics.leading);
 
                         // Mark us as having seen non-whitespace content on this line
                         have_metrics = true;
@@ -559,7 +558,9 @@ impl<'a, B: Brush> BreakLines<'a, B> {
             // Round block/vertical axis metrics
             line.metrics.ascent = line.metrics.ascent.round();
             line.metrics.descent = line.metrics.descent.round();
-            line.metrics.leading = (line.metrics.leading * 0.5).round() * 2.;
+            line.metrics.line_height = line.metrics.line_height.round();
+            line.metrics.leading =
+                line.metrics.line_height - (line.metrics.ascent + line.metrics.descent);
 
             // Compute
             let above = (line.metrics.ascent + line.metrics.leading * 0.5).round();

parley/src/layout/line/mod.rs

@@ -90,6 +90,9 @@ pub struct LineMetrics {
     pub descent: f32,
     /// Typographic leading.
     pub leading: f32,
+    /// The absolute line height (in layout units).
+    /// It matches the CSS definition of line height where it is derived as a multiple of the font size.
+    pub line_height: f32,
     /// Offset to the baseline.
     pub baseline: f32,
     /// Offset for alignment.
@@ -101,9 +104,9 @@ pub struct LineMetrics {
 }
 
 impl LineMetrics {
-    /// Returns the size of the line (ascent + descent + leading).
+    /// Returns the size of the line
     pub fn size(&self) -> f32 {
-        self.ascent + self.descent + self.leading
+        self.line_height
     }
 }
 

parley/src/layout/mod.rs

@@ -197,7 +197,7 @@ pub struct Style<B: Brush> {
     pub underline: Option<Decoration<B>>,
     /// Strikethrough decoration.
     pub strikethrough: Option<Decoration<B>>,
-    /// Multiplicative line height factor.
+    /// Absolute line height in layout units (style line height * font size)
     pub(crate) line_height: f32,
 }
 

parley/src/resolve/mod.rs

@@ -423,7 +423,7 @@ impl<B: Brush> ResolvedStyle<B> {
             brush: self.brush.clone(),
             underline: self.underline.as_layout_decoration(&self.brush),
             strikethrough: self.strikethrough.as_layout_decoration(&self.brush),
-            line_height: self.line_height,
+            line_height: self.line_height * self.font_size,
         }
     }
 }