Merge text runs made identical by style updates

Fixes #452

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/doc/src/tests.rs

@@ -371,3 +371,43 @@ fn custom_easings() {
     assert_eq!(back, Easing::CustomSpring { mass: 2.0, stiffness: 50.0, damping: 5.0 });
     assert_eq!(serde_json::from_value::<Easing>(serde_json::json!("easeInOut")).unwrap(), Easing::EaseInOut);
 }
+
+#[test]
+fn text_style_updates_merge_equal_runs() {
+    let def = TextStyle { font_size: 14.0, ..Default::default() };
+    let mut t = TextData::new("abcd");
+    t.link_text_style(0, 2, &def, "S");
+    t.link_text_style(2, 4, &def, "S");
+    t.restyle(2, 4, |s| s.font_size = 24.0);
+    assert_eq!(t.runs.len(), 2);
+    t.update_text_style("S", &TextStyle { font_size: 24.0, ..def.clone() });
+    let spans = t.spans();
+    assert_eq!(spans.len(), 1);
+    assert_eq!((spans[0].0, spans[0].1), (0, 4));
+    assert_eq!(spans[0].2.font_size, 24.0);
+    assert_eq!(spans[0].2.style_id.as_deref(), Some("S"));
+
+    // Remapping two links to one id merges the runs.
+    let mut t = TextData::new("ab");
+    t.link_text_style(0, 1, &def, "A");
+    t.link_text_style(1, 2, &def, "B");
+    t.map_text_styles(|_| Some("C".into()));
+    assert_eq!(t.spans().len(), 1);
+    assert_eq!(t.style_at(1).style_id.as_deref(), Some("C"));
+
+    // Different values stay separate.
+    let mut t = TextData::new("ab");
+    t.link_text_style(0, 1, &def, "A");
+    t.link_text_style(1, 2, &TextStyle { font_size: 30.0, ..Default::default() }, "B");
+    t.map_text_styles(|_| Some("C".into()));
+    let spans = t.spans();
+    assert_eq!(spans.len(), 2);
+    assert_eq!((spans[0].2.font_size, spans[1].2.font_size), (14.0, 30.0));
+
+    // Unlinking a run equal to the base drops it.
+    let mut t = TextData::new("ab");
+    let base = t.style.clone();
+    t.link_text_style(0, 1, &base, "S");
+    t.map_text_styles(|_| None);
+    assert!(t.runs.is_empty());
+}

crates/doc/src/text.rs

@@ -562,7 +562,7 @@ impl TextData {
         }
     }
 
-    /// Drop empty/out-of-range runs and runs equal to the base style.
+    /// Drop empty/out-of-range runs and runs equal to the base style, and merge touching equal runs.
     pub fn normalize(&mut self) {
         let n = self.char_count();
         let base = self.style.clone();
@@ -571,6 +571,14 @@ impl TextData {
             r.end = r.end.min(n);
         }
         self.runs.sort_by_key(|r| r.start);
+        let mut merged: Vec<StyleRun> = Vec::with_capacity(self.runs.len());
+        for r in std::mem::take(&mut self.runs) {
+            match merged.last_mut() {
+                Some(prev) if prev.end == r.start && prev.style == r.style => prev.end = r.end,
+                _ => merged.push(r),
+            }
+        }
+        self.runs = merged;
     }
 
     /// Replace chars `start..end` with `text`, shifting runs (inserted text takes the style at `start`).