fix: cap session tab title length

src/tab-label.ts

@@ -0,0 +1,22 @@
+/**
+ * Visible label used on editor tabs for a session. Tabs that carry the full
+ * session title grow without bound, pushing sibling tabs out of view, so the
+ * displayed label is capped deterministically. The cap is applied in exactly
+ * the same way for every tab state so activation never changes the length.
+ */
+export const DEFAULT_TAB_LABEL_MAX = 28;
+
+function sliceCodePoints(value: string, maxLength: number): string {
+  const codePoints = Array.from(value);
+  return codePoints.slice(0, maxLength).join("");
+}
+
+/** Collapse whitespace, cap by code points, and add an ellipsis when cut. */
+export function limitTabLabel(
+  label: string,
+  maxLength: number = DEFAULT_TAB_LABEL_MAX,
+): string {
+  const compact = label.replace(/\s+/g, " ").trim();
+  if (Array.from(compact).length <= maxLength) { return compact; }
+  return `${sliceCodePoints(compact, maxLength).replace(/[\s.…]+$/u, "")}…`;
+}

src/test/tab-label.test.ts

@@ -0,0 +1,49 @@
+import * as assert from "node:assert";
+import { readFileSync } from "node:fs";
+import { limitTabLabel } from "../tab-label.js";
+
+suite("Session tab label limits", () => {
+  test("keeps short labels unchanged", () => {
+    assert.strictEqual(limitTabLabel("Fix sidebar refresh"), "Fix sidebar refresh");
+    assert.strictEqual(limitTabLabel("Pi"), "Pi");
+    assert.strictEqual(limitTabLabel(""), "");
+  });
+
+  test("truncates long labels to the configured maximum with an ellipsis", () => {
+    const label = "a very long session title that would push sibling tabs out of view";
+    const limited = limitTabLabel(label, 28);
+    assert.strictEqual(Array.from(limited).length, 29);
+    assert.ok(limited.endsWith("…"));
+    assert.strictEqual(Array.from(limited.slice(0, -1)).length, 28);
+  });
+
+  test("counts code points instead of UTF-16 code units", () => {
+    const exactLengthLabel = "🤖".repeat(28);
+    assert.strictEqual(limitTabLabel(exactLengthLabel, 28), exactLengthLabel);
+
+    const label = "🤖".repeat(40) + " title";
+    const limited = limitTabLabel(label, 28);
+    assert.strictEqual(Array.from(limited).length, 29);
+    assert.ok(!limited.includes("\uFFFD"), "must not split surrogate pairs");
+  });
+
+  test("collapses whitespace before applying the cap", () => {
+    const label = "标题  很 长  占用  空间 ".repeat(4) + "标题栏";
+    const limited = limitTabLabel(label, 28);
+    assert.ok(!limited.includes("  "), "consecutive spaces must be collapsed");
+    assert.ok(limited.endsWith("…"));
+  });
+
+  test("caps every tab state through the shared title builder", () => {
+    const source = readFileSync(
+      new URL("../../src/webview-panel.ts", import.meta.url),
+      "utf8",
+    );
+    assert.match(source, /const label = limitTabLabel\(this\._tabSummary \?\? "Pi"\);/);
+    assert.match(
+      source,
+      /this\.panel\.title = \(this\._tabStreaming \? "\\u25CF " : "\\u25CB "\) \+ label;/,
+    );
+    assert.match(source, /import \{ limitTabLabel \} from "\.\/tab-label\.js";/);
+  });
+});

src/webview-panel.ts

@@ -11,6 +11,7 @@ import { mergeInitialHistoryEvents } from "./history-event-sync.js";
 import { SessionCapabilitySnapshot } from "./capability-snapshot.js";
 import { piError } from "./logger.js";
 import { getWorkspaceCwd } from "./workspace-context.js";
+import { limitTabLabel } from "./tab-label.js";
 import type { PiService } from "./pi-service.js";
 import type { PiServiceEvent } from "./types.js";
 import {
@@ -860,8 +861,9 @@ export class PiWebviewPanel {
       return;
     }
 
-    const label = this._tabSummary ?? "Pi";
-    // Bullet prefix: ● busy, ○ idle — consistent with status bar
+    const label = limitTabLabel(this._tabSummary ?? "Pi");
+    // Bullet prefix: ● busy, ○ idle — consistent with status bar. The cap is
+    // applied before the prefix so streaming and idle tabs keep equal length.
     this.panel.title = (this._tabStreaming ? "\u25CF " : "\u25CB ") + label;
   }