feat: try to give a proper advanced width for varaible font

src/metrics.rs

@@ -118,12 +118,25 @@ impl Font {
  y_max: units as i16,
  })
  })?;
+ let lsb = f.glyph_hor_side_bearing(glyph_id).unwrap_or(0);
+ let aw = f.glyph_hor_advance(glyph_id)?;
+ // FIXME: for variable fonts, this is not the same with harfbuzz, mainly
+ // because of phantom points not implemented in ttf-parser. More on this issue:
+ // https://github.com/RazrFalcon/rustybuzz/issues/91#issuecomment-1951036351
+ // here we simply copy rustybuzz's logic to ensure the "wrong" advance is always
+ // larger than the "actual" width, which could add some space to texts, but
+ // at least enough room.
+ let advanced_x = if f.is_variable() && f.has_non_default_variation_coordinates() {
+ (bbox.x_max + bbox.x_min) as u16
+ } else {
+ aw
+ };
  Some(CharMetrics {
  c,
  glyph_id,
- advanced_x: f.glyph_hor_advance(glyph_id)?,
+ advanced_x,
  bbox,
- lsb: f.glyph_hor_side_bearing(glyph_id).unwrap_or(0),
+ lsb,
  units,
  height,
  missing: false,