Bug: /rewind via picker crashes TUI (orphaned widget mount after _remount_current_session)

[Dz-hy]

Bug Description

Using /rewind without arguments (which shows a picker to select the number of turns) causes the TUI to flash errors and immediately exit. Using /rewind <n> with a numeric argument works fine.

Reproduction Steps

1. Start tuiapp_v2.py with a conversation that has multiple turns
2. Type /rewind (no argument) to open the turn-selection picker
3. Select any number of turns from the picker (e.g. "回退 1 轮 · ...")
4. TUI crashes with a flash of errors and exits

Root Cause

The crash occurs in _collapse_choice() (line ~2608) after the on_select callback returns.

The call chain:

1. User selects a rewind option → _collapse_choice(msg, idx) is called
2. Line 2628: msg.on_select(value) calls _do_rewind(n) (inside a try/except)
3. Inside _do_rewind:
   • Line 2815: sess.messages = sess.messages[:real_user[-n]] — truncates messages, removing the choice message itself from sess.messages
   • Line 2818: self._remount_current_session() → container.remove_children() — removes ALL widgets from the DOM
4. Back in _collapse_choice (lines 2631-2644, outside the try/except):

   anchor = msg._hint_widget or msg._body_widget          # points to a widget removed from DOM
   container.mount(new_widget, after=anchor)                # 💥 CRASH: anchor is no longer a child of container

The try/except at lines 2627-2630 only wraps the on_select call itself. The subsequent widget manipulation at lines 2631-2644 is unprotected and raises an unhandled exception, crashing the TUI.

Why /rewind <n> works but /rewind doesn't

• /rewind <n> (line 2769): calls self._system(self._do_rewind(n)) directly — no widget replacement happens after
• /rewind (no arg, line 2780-2785): creates a kind="choice" message with on_select=lambda v: self._do_rewind(v) — after _do_rewind returns, _collapse_choice tries to replace widgets that were already destroyed by _remount_current_session()

Suggested Fix

In _collapse_choice, after calling on_select, check if msg was removed from sess.messages. If so, skip the widget replacement since _do_rewind already handled the UI update:

# After line 2630, before line 2631, add:
sess = self.sessions.get(self.current_id)
if sess and msg not in sess.messages:
    # on_select callback removed this message (e.g. rewind) — skip widget replacement
    return

Or alternatively, wrap lines 2631-2651 in a try/except to prevent the crash.

Environment

• OS: Windows
• Python: 3.x
• Textual: latest
• File: frontends/tuiapp_v2.py, _collapse_choice() ~line 2608, _do_rewind() ~line 2803