feat: support configurable tabline status label

src/session.rs

@@ -220,6 +220,7 @@ impl EditorSession {
             ));
         }
         editor.set_diff_context_options(config.diff_context_lines, config.diff_expand_lines);
+        editor.set_tabline_label(config.tabline_label);
         let mut plugins = BTreeMap::new();
         for configured in config.plugins {
             let path = configured.path;
@@ -756,6 +757,11 @@ impl EditorSession {
                     }
                     UpdateEffect::Completion { .. } => {}
                     UpdateEffect::Message { .. } => {}
+                    UpdateEffect::TablineLabel { text } => {
+                        if text.len() > 4 * 1024 * 1024 {
+                            return Err("Tabline label exceeds 4 MiB".into());
+                        }
+                    }
                 }
             }
             Ok(())
@@ -851,6 +857,9 @@ impl EditorSession {
                     changed |= self.editor.message() != text;
                     self.editor.set_message(text);
                 }
+                UpdateEffect::TablineLabel { text } => {
+                    changed |= self.editor.set_tabline_label(text);
+                }
             }
         }
         self.editor.refresh_live_diff();
@@ -878,13 +887,19 @@ impl EditorSession {
         }
         if effects
             .iter()
-            .all(|effect| matches!(effect, Effect::Message(_)))
+            .all(|effect| matches!(effect, Effect::Message(_) | Effect::TablineLabel(_)))
         {
             let mut changed = false;
             for effect in effects {
-                if let Effect::Message(text) = effect {
-                    changed |= self.editor.message() != text;
-                    self.editor.set_message(text);
+                match effect {
+                    Effect::Message(text) => {
+                        changed |= self.editor.message() != text;
+                        self.editor.set_message(text);
+                    }
+                    Effect::TablineLabel(text) => {
+                        changed |= self.editor.set_tabline_label(text);
+                    }
+                    _ => unreachable!("presentation-only effects checked above"),
                 }
             }
             return Ok(changed);
@@ -2980,6 +2995,70 @@ end
         drain_until(session, |session| session.editor().message() == delivered);
     }
 
+    #[test]
+    fn tabline_updates_survive_tab_switches_and_remain_atomic_in_readonly_views() {
+        let fixture = Fixture::new();
+        let mut session = fixture.asynchronous(None);
+        publish(
+            &mut session,
+            vec![serde_json::json!({
+                "type": "tabline_label", "text": "bat 37%"
+            })],
+        );
+        drain_until(&mut session, |session| {
+            session.editor().tabline_label() == "bat 37%"
+        });
+        let gate = fixture.0.join("label.gate");
+        hold(
+            &mut session,
+            &gate,
+            vec![serde_json::json!({
+                "type": "tabline_label", "text": "bat 38%"
+            })],
+        );
+        barrier(&mut session, "label request received");
+        session.execute("tab.new");
+        release(&mut session, &gate);
+        assert_eq!(session.editor().tabline_label(), "bat 38%");
+        session.execute("tab.previous");
+        assert_eq!(session.editor().tabline_label(), "bat 38%");
+        let update = |text: &str| PluginUpdate {
+            request_id: None,
+            effects: vec![UpdateEffect::TablineLabel { text: text.into() }],
+        };
+        assert!(
+            !session
+                .apply_update("generic_fixture", update("bat 38%"))
+                .unwrap()
+        );
+        let rejected = PluginUpdate {
+            request_id: None,
+            effects: vec![
+                UpdateEffect::TablineLabel {
+                    text: "must not leak".into(),
+                },
+                UpdateEffect::SemanticTokens {
+                    buffer_id: session.editor().buffer_id(),
+                    revision: session.editor().text_revision(),
+                    tokens: None,
+                },
+            ],
+        };
+        assert!(session.apply_update("generic_fixture", rejected).is_err());
+        assert_eq!(session.editor().tabline_label(), "bat 38%");
+        session.editor.execute("diff");
+        assert!(session.editor.mutation_blocked());
+        assert!(
+            session
+                .apply_update("generic_fixture", update("bat\t39%\n"))
+                .unwrap()
+        );
+        assert_eq!(session.editor().tabline_label(), "bat�39%�");
+        assert_eq!(session.editor().lines(), [""]);
+        assert!(session.apply_update("generic_fixture", update("")).unwrap());
+        assert_eq!(session.editor().tabline_label(), "");
+    }
+
     #[test]
     fn async_documents_include_unnamed_hidden_versions_real_saves_renames_and_closes() {
         let fixture = Fixture::new();