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/prompt-display.ts

@@ -0,0 +1,39 @@
+const graphemes = new Intl.Segmenter(undefined, { granularity: "grapheme" })
+
+function displayOffsetIndex(value: string, offset: number) {
+  if (offset <= 0) return 0
+
+  let width = 0
+  for (const part of graphemes.segment(value)) {
+    const next = width + Bun.stringWidth(part.segment)
+    if (next > offset) return part.index
+    width = next
+  }
+
+  return value.length
+}
+
+export function displaySlice(value: string, start = 0, end = Bun.stringWidth(value)) {
+  return value.slice(displayOffsetIndex(value, start), displayOffsetIndex(value, end))
+}
+
+export function displayCharAt(value: string, offset: number) {
+  let width = 0
+  for (const part of graphemes.segment(value)) {
+    const next = width + Bun.stringWidth(part.segment)
+    if (offset === width || offset < next) return part.segment
+    width = next
+  }
+}
+
+export function mentionTriggerIndex(value: string, offset = Bun.stringWidth(value)) {
+  const text = displaySlice(value, 0, offset)
+  const index = text.lastIndexOf("@")
+  if (index === -1) return
+
+  const before = index === 0 ? undefined : text[index - 1]
+  const query = text.slice(index)
+  if ((before === undefined || /\s/.test(before)) && !/\s/.test(query)) {
+    return Bun.stringWidth(text.slice(0, index))
+  }
+}