fix(shared): truncate no longer splits surrogate pairs

packages/shared/src/String.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import { truncate } from "./String.ts";
+
+describe("truncate", () => {
+  it("cuts before a surrogate pair that would be split", () => {
+    expect(truncate("abcd😀 more", 5)).toBe("abcd...");
+  });
+
+  it("keeps a surrogate pair that fits exactly", () => {
+    expect(truncate("abc😀 more", 5)).toBe("abc😀...");
+  });
+
+  it("keeps ASCII and BMP truncation lengths", () => {
+    expect(truncate("  hello world  ", 5)).toBe("hello...");
+    expect(truncate("日本語のテキスト", 3)).toBe("日本語...");
+    expect(truncate("short", 5)).toBe("short");
+  });
+});

packages/shared/src/String.ts

@@ -1,8 +1,19 @@
+const isHighSurrogate = (code: number) => code >= 0xd800 && code <= 0xdbff;
+const isLowSurrogate = (code: number) => code >= 0xdc00 && code <= 0xdfff;
+
 export function truncate(text: string, maxLength = 50): string {
   const trimmed = text.trim();
   if (trimmed.length <= maxLength) {
     return trimmed;
   }
 
-  return `${trimmed.slice(0, maxLength)}...`;
+  let head = trimmed.slice(0, maxLength);
+  // Cutting between the halves of a surrogate pair leaves a lone surrogate that renders as U+FFFD.
+  if (
+    isHighSurrogate(head.charCodeAt(head.length - 1)) &&
+    isLowSurrogate(trimmed.charCodeAt(head.length))
+  ) {
+    head = head.slice(0, -1);
+  }
+  return `${head}...`;
 }