Make empty ranges link and unlink text styles on all characters

Fixes #453

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 +371,25 @@ 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_empty_range_links_and_unlinks_all() {
+    let def = TextStyle { font_style: "Bold".into(), ..Default::default() };
+    let mut t = TextData::new("abcd");
+    t.link_text_style(2, 2, &def, "S");
+    assert!((0..4).all(|i| t.style_at(i).style_id.as_deref() == Some("S") && t.style_at(i).font_style == "Bold"));
+    t.unlink_text_style(2, 2);
+    assert!((0..4).all(|i| t.style_at(i).style_id.is_none() && t.style_at(i).font_style == "Bold"));
+    // Partial ranges stay partial.
+    t.link_text_style(0, 4, &def, "S");
+    t.unlink_text_style(1, 3);
+    assert_eq!(t.style_at(0).style_id.as_deref(), Some("S"));
+    assert!(t.style_at(1).style_id.is_none() && t.style_at(2).style_id.is_none());
+    assert_eq!(t.style_at(3).style_id.as_deref(), Some("S"));
+    // Empty text changes the base style.
+    let mut e = TextData::new("");
+    e.link_text_style(0, 0, &def, "S");
+    assert_eq!(e.style.style_id.as_deref(), Some("S"));
+    e.unlink_text_style(0, 0);
+    assert!(e.style.style_id.is_none());
+}

crates/doc/src/text.rs

@@ -675,11 +675,13 @@ impl TextData {
     /// Link chars `start..end` (all of them when the range is empty or the whole text) to text
     /// style `id`, defined as `def`.
     pub fn link_text_style(&mut self, start: usize, end: usize, def: &TextStyle, id: &str) {
+        let (start, end) = if start == end { (0, self.char_count()) } else { (start, end) };
         self.restyle(start, end, |s| s.take_text_style(def, id));
     }
 
     /// Unlink chars `start..end` (all when empty or whole) from their text styles; values stay.
     pub fn unlink_text_style(&mut self, start: usize, end: usize) {
+        let (start, end) = if start == end { (0, self.char_count()) } else { (start, end) };
         self.restyle(start, end, |s| s.style_id = None);
     }