Calculate an overshoot when mousing to build columnar selections

crates/gpui/src/font_cache.rs

@@ -157,6 +157,17 @@ impl FontCache {
         bounds.width() * self.em_scale(font_id, font_size)
     }
 
+    pub fn em_advance(&self, font_id: FontId, font_size: f32) -> f32 {
+        let glyph_id;
+        let advance;
+        {
+            let state = self.0.read();
+            glyph_id = state.fonts.glyph_for_char(font_id, 'm').unwrap();
+            advance = state.fonts.advance(font_id, glyph_id).unwrap();
+        }
+        advance.x() * self.em_scale(font_id, font_size)
+    }
+
     pub fn line_height(&self, font_id: FontId, font_size: f32) -> f32 {
         let height = self.metric(font_id, |m| m.bounding_box.height());
         (height * self.em_scale(font_id, font_size)).ceil()

crates/gpui/src/fonts.rs

@@ -151,6 +151,10 @@ impl TextStyle {
         font_cache.em_width(self.font_id, self.font_size)
     }
 
+    pub fn em_advance(&self, font_cache: &FontCache) -> f32 {
+        font_cache.em_advance(self.font_id, self.font_size)
+    }
+
     pub fn descent(&self, font_cache: &FontCache) -> f32 {
         font_cache.metric(self.font_id, |m| m.descent) * self.em_scale(font_cache)
     }

crates/gpui/src/platform.rs

@@ -148,6 +148,7 @@ pub trait FontSystem: Send + Sync {
     ) -> anyhow::Result<FontId>;
     fn font_metrics(&self, font_id: FontId) -> FontMetrics;
     fn typographic_bounds(&self, font_id: FontId, glyph_id: GlyphId) -> anyhow::Result<RectF>;
+    fn advance(&self, font_id: FontId, glyph_id: GlyphId) -> anyhow::Result<Vector2F>;
     fn glyph_for_char(&self, font_id: FontId, ch: char) -> Option<GlyphId>;
     fn rasterize_glyph(
         &self,

crates/gpui/src/platform/mac/fonts.rs

@@ -69,6 +69,10 @@ impl platform::FontSystem for FontSystem {
         self.0.read().typographic_bounds(font_id, glyph_id)
     }
 
+    fn advance(&self, font_id: FontId, glyph_id: GlyphId) -> anyhow::Result<Vector2F> {
+        self.0.read().advance(font_id, glyph_id)
+    }
+
     fn glyph_for_char(&self, font_id: FontId, ch: char) -> Option<GlyphId> {
         self.0.read().glyph_for_char(font_id, ch)
     }
@@ -137,6 +141,10 @@ impl FontSystemState {
         Ok(self.fonts[font_id.0].typographic_bounds(glyph_id)?)
     }
 
+    fn advance(&self, font_id: FontId, glyph_id: GlyphId) -> anyhow::Result<Vector2F> {
+        Ok(self.fonts[font_id.0].advance(glyph_id)?)
+    }
+
     fn glyph_for_char(&self, font_id: FontId, ch: char) -> Option<GlyphId> {
         self.fonts[font_id.0].glyph_for_char(ch)
     }