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/src/cli/cmd/tui/component/prompt/autocomplete.tsx

@@ -20,6 +20,7 @@ import { useFrecency } from "./frecency"
 import { useBindings } from "../../keymap"
 import { Reference } from "@/reference/reference"
 import type { Config } from "@/config/config"
+import { displayCharAt, mentionTriggerIndex } from "@/cli/cmd/prompt-display"
 
 function removeLineRange(input: string) {
   const hashIndex = input.lastIndexOf("#")
@@ -159,7 +160,7 @@ export function Autocomplete(props: {
     const input = props.input()
     const currentCursorOffset = input.cursorOffset
 
-    const charAfterCursor = props.value.at(currentCursorOffset)
+    const charAfterCursor = displayCharAt(props.value, currentCursorOffset)
     const needsSpace = charAfterCursor !== " "
     const append = "@" + text + (needsSpace ? " " : "")
 
@@ -787,13 +788,8 @@ export function Autocomplete(props: {
         }
 
         // Check for "@" trigger - find the nearest "@" before cursor with no whitespace between
-        const text = value.slice(0, offset)
-        const idx = text.lastIndexOf("@")
-        if (idx === -1) return
-
-        const between = text.slice(idx)
-        const before = idx === 0 ? undefined : value[idx - 1]
-        if ((before === undefined || /\s/.test(before)) && !between.match(/\s/)) {
+        const idx = mentionTriggerIndex(value, offset)
+        if (idx !== undefined) {
           show("@")
           setStore("index", idx)
         }