Use CSS compatible line-height computation.

Let line-height be line_height * font_size with no regard for font metrics.
Font metrics are still used to determine the baseline with the line box.

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,
  }
  }
 }