fix: TUI crash with Unicode characters in branch selector

- Use truncateToWidth instead of substring in user-message-selector.ts
- Fix truncateToWidth to use Intl.Segmenter for proper grapheme handling
- Add tests for Unicode truncation behavior

packages/coding-agent/test/truncate-to-width.test.ts

@@ -0,0 +1,81 @@
+import { truncateToWidth, visibleWidth } from "@mariozechner/pi-tui";
+import { describe, expect, it } from "vitest";
+
+/**
+ * Tests for truncateToWidth behavior with Unicode characters.
+ *
+ * These tests verify that truncateToWidth properly handles text with
+ * Unicode characters that have different byte vs display widths.
+ */
+describe("truncateToWidth", () => {
+	it("should truncate messages with Unicode characters correctly", () => {
+		// This message contains a checkmark (✔) which may have display width > 1 byte
+		const message = '✔ script to run › dev $ concurrently "vite" "node --import tsx ./';
+		const width = 67;
+		const maxMsgWidth = width - 2; // Account for cursor
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+		const truncatedWidth = visibleWidth(truncated);
+
+		expect(truncatedWidth).toBeLessThanOrEqual(maxMsgWidth);
+	});
+
+	it("should handle emoji characters", () => {
+		const message = "🎉 Celebration! 🚀 Launch 📦 Package ready for deployment now";
+		const width = 40;
+		const maxMsgWidth = width - 2;
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+		const truncatedWidth = visibleWidth(truncated);
+
+		expect(truncatedWidth).toBeLessThanOrEqual(maxMsgWidth);
+	});
+
+	it("should handle mixed ASCII and wide characters", () => {
+		const message = "Hello 世界 Test 你好 More text here that is long";
+		const width = 30;
+		const maxMsgWidth = width - 2;
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+		const truncatedWidth = visibleWidth(truncated);
+
+		expect(truncatedWidth).toBeLessThanOrEqual(maxMsgWidth);
+	});
+
+	it("should not truncate messages that fit", () => {
+		const message = "Short message";
+		const width = 50;
+		const maxMsgWidth = width - 2;
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+
+		expect(truncated).toBe(message);
+		expect(visibleWidth(truncated)).toBeLessThanOrEqual(maxMsgWidth);
+	});
+
+	it("should add ellipsis when truncating", () => {
+		const message = "This is a very long message that needs to be truncated";
+		const width = 30;
+		const maxMsgWidth = width - 2;
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+
+		expect(truncated).toContain("...");
+		expect(visibleWidth(truncated)).toBeLessThanOrEqual(maxMsgWidth);
+	});
+
+	it("should handle the exact crash case from issue report", () => {
+		// Terminal width was 67, line had visible width 68
+		// The problematic text contained "✔" and "›" characters
+		const message = '✔ script to run › dev $ concurrently "vite" "node --import tsx ./server.ts"';
+		const terminalWidth = 67;
+		const cursorWidth = 2; // "› " or "  "
+		const maxMsgWidth = terminalWidth - cursorWidth;
+
+		const truncated = truncateToWidth(message, maxMsgWidth);
+		const finalWidth = visibleWidth(truncated);
+
+		// The final line (cursor + message) must not exceed terminal width
+		expect(finalWidth + cursorWidth).toBeLessThanOrEqual(terminalWidth);
+	});
+});