fix(copy): measure the characters at a wrap by code point, not UTF-16 unit

isWide looked at a single UTF-16 code unit, so an astral character at the
break (CJK Ext-B, emoji) was a lone surrogate of width 1 and the rejoin
inserted a space between two wide characters.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src/components/terminal/copyCleanup.test.ts

@@ -208,6 +208,14 @@ describe("rejoinHardWraps", () => {
     expect(rejoinHardWraps(`${a}\n  の続き`, { cols: 69 })).toBe(`${a}の続き`);
   });
 
+  it("joins astral wide characters (CJK Ext-B) without inserting a space", () => {
+    const a = "  " + "\u{20000}".repeat(33); // 2 + 66 = 68 cells
+    expect(cellWidth(a)).toBe(68);
+    expect(rejoinHardWraps(`${a}\n  \u{20001}\u{20002}`, { cols: 69 })).toBe(`${a}\u{20001}\u{20002}`);
+    const e = "  " + "\u{1F600}".repeat(33); // emoji, also two cells each
+    expect(rejoinHardWraps(`${e}\n  \u{1F601}`, { cols: 69 })).toBe(`${e}\u{1F601}`);
+  });
+
   it("uses the selection's start column for a first line that began mid-row", () => {
     // Selection started at column 18, so the first line is only 51
     // characters but ended at column 69 of its row.

app/src/components/terminal/copyCleanup.ts

@@ -172,7 +172,10 @@ function toRow(text: string, offset: number): Row {
 
 const lastToken = (s: string) => s.slice(s.lastIndexOf(" ") + 1);
 const firstToken = (s: string) => s.split(" ", 1)[0];
-const isWide = (ch: string | undefined) => !!ch && charWidth(ch) === 2;
+/** First / last *code point* — an astral character (CJK Ext-B, most emoji) is two UTF-16 units. */
+const firstChar = (s: string) => (s ? String.fromCodePoint(s.codePointAt(0)!) : "");
+const lastChar = (s: string) => Array.from(s.slice(-2)).pop() ?? "";
+const isWide = (ch: string) => !!ch && charWidth(ch) === 2;
 
 /**
  * Re-joins rows that Claude Code broke to fit the terminal width.
@@ -235,8 +238,7 @@ function joinerFor(prev: Row, cur: Row, head: Row, headPartial: boolean, cols: n
   const next = firstToken(cur.body);
   if (prev.end + 1 + cellWidth(next) <= width) return null;
 
-  const prevLast = prev.body[prev.body.length - 1];
-  if (isWide(prevLast) && isWide(cur.body[0])) return "";
+  if (isWide(lastChar(prev.body)) && isWide(firstChar(cur.body))) return "";
   if (prev.end === width && cellWidth(lastToken(prev.body)) + cellWidth(next) > width - cur.lead) {
     return "";
   }