fix(tui): right-click copies selection, only pastes when no selection

Sub-issue 5 of #22034.

Right-click on the composer always pasted from the clipboard, even when
the user had highlighted text — diverging from terminal-native behavior
(xterm/iTerm/gnome-terminal) where right-click copies an active selection
and only pastes when nothing is selected.

Extract a small pure helper, decideRightClickAction(value, range), and
route the existing onMouseDown right-click branch through it. Selection
present and non-empty -> writeClipboardText(slice). Otherwise fall back
to the existing emitPaste path.

ui-tui/src/__tests__/textInputRightClick.test.ts

@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+
+import { decideRightClickAction } from '../components/textInput.js'
+
+describe('decideRightClickAction', () => {
+  it('returns paste when there is no selection', () => {
+    expect(decideRightClickAction('hello world', null)).toEqual({ action: 'paste' })
+  })
+
+  it('returns paste for a collapsed (empty) range', () => {
+    expect(decideRightClickAction('hello world', { end: 5, start: 5 })).toEqual({
+      action: 'paste'
+    })
+  })
+
+  it('copies the slice when range covers non-empty text', () => {
+    expect(decideRightClickAction('hello world', { end: 5, start: 0 })).toEqual({
+      action: 'copy',
+      text: 'hello'
+    })
+  })
+
+  it('copies a middle slice', () => {
+    expect(decideRightClickAction('hello world', { end: 11, start: 6 })).toEqual({
+      action: 'copy',
+      text: 'world'
+    })
+  })
+
+  it('falls back to paste when slice is empty (out-of-range indices)', () => {
+    expect(decideRightClickAction('', { end: 5, start: 0 })).toEqual({ action: 'paste' })
+  })
+
+  it('handles unicode (emoji, CJK) in the slice', () => {
+    const value = 'hi 你好 🎉'
+    expect(decideRightClickAction(value, { end: 5, start: 3 })).toEqual({
+      action: 'copy',
+      text: '你好'
+    })
+  })
+
+  it('preserves leading/trailing whitespace in the copied slice', () => {
+    expect(decideRightClickAction('  spaced  ', { end: 10, start: 0 })).toEqual({
+      action: 'copy',
+      text: '  spaced  '
+    })
+  })
+})

ui-tui/src/components/textInput.tsx

@@ -970,10 +970,15 @@ export function TextInput({
           return
         }
 
-        // Right-click → route through the same path as Alt+V so the composer
-        // clipboard RPC (text or image) handles it.
+        // Right-click → copy active selection if any, otherwise paste.
         if (e.button === 2) {
           e.stopImmediatePropagation?.()
+          const decision = decideRightClickAction(vRef.current, selRange())
+          if (decision.action === 'copy') {
+            void writeClipboardText(decision.text)
+
+            return
+          }
           emitPaste({ cursor: curRef.current, hotkey: true, text: '', value: vRef.current })
 
           return
@@ -1045,6 +1050,34 @@ interface TextInputProps {
   voiceRecordKey?: ParsedVoiceRecordKey
 }
 
+export type RightClickDecision =
+  | { action: 'copy'; text: string }
+  | { action: 'paste' }
+
+/**
+ * Decide what right-click should do on the composer:
+ *   - non-empty selection → copy that text to the clipboard
+ *   - no selection (or empty/collapsed range) → fall through to paste
+ *
+ * Mirrors terminal-native behavior (xterm, iTerm, gnome-terminal) where
+ * right-click pastes only when there is nothing selected to copy.
+ *
+ * Callers pass the already-normalized range from `selRange()` (start <= end,
+ * or null when collapsed), so this helper does not need to re-normalize.
+ */
+export function decideRightClickAction(
+  value: string,
+  range: { end: number; start: number } | null
+): RightClickDecision {
+  if (range && range.end > range.start) {
+    const text = value.slice(range.start, range.end)
+    if (text) {
+      return { action: 'copy', text }
+    }
+  }
+  return { action: 'paste' }
+}
+
 export const shouldPassThroughToGlobalHandler = (
   input: string,
   key: Key,