App hangs when right-clicking a chat/session with multiple open

### Describe the bug

## What happened
With multiple chats/sessions open, right-clicking any of them freezes the app (unresponsive).

### Affected version

1.0.64-0

### Steps to reproduce the behavior

## Steps to reproduce
1. Open several chats and/or project sessions
2. Right-click any one in the sidebar
3. App hangs

### Expected behavior

## Expected
A context menu appears; app stays responsive.

### Additional context

## Environment
- OS: macOS (Darwin)
- Copilot CLI/app version: 1.0.64-0
- Frequency: every time

Start by reproducing the hang on macOS with several chats or project sessions open, then right-click a sidebar item and observe the context-menu path. The issue names no source file, test, or entry point, so trace the sidebar session context-menu handling from that interaction. Done means the menu appears and the app remains responsive every time.

索引モデルが issue の本文から書いたものです。