prompt: fix cursor math for wide characters (#27017)

String.length counts code points, not display columns, so CJK
characters and emoji that occupy two terminal cells caused
misaligned cursors, broken mention triggers, and incorrect
history restoration offsets.

Use Bun.stringWidth for now, we need an alternative for this.

Fix #26716
Close #26922

packages/opencode/test/cli/run/prompt.shared.test.ts

@@ -1,8 +1,11 @@
 import { describe, expect, test } from "bun:test"
 import {
   createPromptHistory,
+  displayCharAt,
+  displaySlice,
   isExitCommand,
   isNewCommand,
+  mentionTriggerIndex,
   movePromptHistory,
   printableBinding,
   promptCycle,
@@ -85,6 +88,53 @@ describe("run prompt shared", () => {
     expect(draft.state.index).toBeNull()
   })
 
+  test("uses display-width cursors for history restoration", () => {
+    const base = createPromptHistory([prompt("one"), prompt("中文")])
+
+    const latest = movePromptHistory(base, -1, "草稿", 0)
+    expect(latest.apply).toBe(true)
+    expect(latest.text).toBe("中文")
+    expect(latest.cursor).toBe(0)
+
+    const older = movePromptHistory(latest.state, -1, "中文", 0)
+    expect(older.apply).toBe(true)
+    expect(older.text).toBe("one")
+    expect(older.cursor).toBe(0)
+
+    const newer = movePromptHistory(older.state, 1, "one", Bun.stringWidth("one"))
+    expect(newer.apply).toBe(true)
+    expect(newer.text).toBe("中文")
+    expect(newer.cursor).toBe(Bun.stringWidth("中文"))
+
+    const draft = movePromptHistory(newer.state, 1, "中文", Bun.stringWidth("中文"))
+    expect(draft.apply).toBe(true)
+    expect(draft.text).toBe("草稿")
+    expect(draft.cursor).toBe(Bun.stringWidth("草稿"))
+  })
+
+  test("uses display-width offsets for mention helpers", () => {
+    expect(mentionTriggerIndex("@")).toBe(0)
+    expect(mentionTriggerIndex("test @")).toBe(5)
+    expect(mentionTriggerIndex("中文 @")).toBe(5)
+    expect(mentionTriggerIndex("こんにちは @")).toBe(11)
+    expect(mentionTriggerIndex("한국어 @")).toBe(7)
+    expect(mentionTriggerIndex("🙂 @")).toBe(3)
+    expect(mentionTriggerIndex("中文 @src file", Bun.stringWidth("中文 @src"))).toBe(5)
+    expect(displayCharAt("中文 @src", Bun.stringWidth("中文 @"))).toBe("s")
+    expect(displaySlice("中文 @src", 5, Bun.stringWidth("中文 @src"))).toBe("@src")
+    expect(displaySlice("中文 @src", 6, Bun.stringWidth("中文 @src"))).toBe("src")
+    expect(mentionTriggerIndex("👨‍👩‍👧‍👦 @src", Bun.stringWidth("👨‍👩‍👧‍👦 @src"))).toBe(3)
+    expect(displayCharAt("👨‍👩‍👧‍👦 @src", Bun.stringWidth("👨‍👩‍👧‍👦 @"))).toBe("s")
+    expect(displaySlice("👨‍👩‍👧‍👦 @src", 3, Bun.stringWidth("👨‍👩‍👧‍👦 @src"))).toBe("@src")
+    expect(mentionTriggerIndex("中文@")).toBeUndefined()
+    expect(mentionTriggerIndex("こんにちは@")).toBeUndefined()
+    expect(mentionTriggerIndex("한국어@")).toBeUndefined()
+    expect(mentionTriggerIndex("🙂@")).toBeUndefined()
+    expect(mentionTriggerIndex("hello@")).toBeUndefined()
+    expect(mentionTriggerIndex("foo@bar.com")).toBeUndefined()
+    expect(mentionTriggerIndex("中文 @src file")).toBeUndefined()
+  })
+
   test("handles direct and leader-based variant cycling", () => {
     const keys = promptKeys(keybinds)