Markdown list rendering hangs the chat window

[classicsc]

Describe the bug The chat window freezes when rendering a message containing a deeply nested markdown list. Shorter lists cause a temporary hang that recovers, while very large/deep lists seem to be unrecoverable. The chat window is unusable until I force-quit "GitHub Copilot for Xcode Extension" in Activity Monitor and delete the conversation history with rm ~/.config/github-copilot/xcode/*/conversations/*.db (it hangs on startup, so restarting xcode doesn't help unless the database is deleted). I ran into this trying to send some detailed instructions to Agent mode, but it happens in the Ask tab as well.

Versions

• Copilot for Xcode: 0.40.0
• Xcode: 16.4
• macOS: 15.6

Steps to reproduce

1. Open chat from the menubar icon.
2. Send the following message (Agent or Ask tab):

Here's a list:

* List item
    * List item
      * List item
      * List item
        * List item
          * List item
        * List item
          * List item
            * List item
            * List item
  * List item
  * List item
    * List item
      * List item
      * List item
        * List item
          * List item
        * List item
          * List item
            * List item

On my hardware, the above causes it to hang for about a minute but recovers eventually (see the video). Duplicate the list 3x, and it seems to hang forever.

3. Observe that the chat window freezes until the message is rendered (the response is apparently still happening, I saw agent mode opening files and making edits while it was frozen). The menubar icon also becomes unresponsive.
4. Force-quit the extension and restart Xcode.
5. Re-open the chat window: the freeze reoccurs.

Screenshots

https://github.com/user-attachments/assets/46daeee3-d564-40c5-b358-f813435b7b8e

It doesn't show in the recording, but the cursor becomes the spinning beach ball.

Logs I turned on verbose logging but there were no interesting messages in the log from the time I tried reproducing it.

Additional context Hardware: MacBook Pro with M1 Pro, 16GB memory.

While troubleshooting, I deleted the ~/Library/Application Support/com.github.CopilotForXcode and ~/.config/github-copilot directories and reinstalled from homebrew, but the issue persisted if I sent a problematic message again.

[testforstephen]

thanks for reporting the issue. I can reproduce it. The markdown engine has performance issue to render such kind of markdown text. let's see how to improve it either the UI side or the markdown engine itself.