Agents window: Can't seem to select certain messages in chat nav bar

Testing #332192

* Launch the Agents window
* Expand the "+" action on for the `vscode-internalbacklog` workspace and choose "New Session from Pull Request"
* Pick a pull request, and create a session for it. Use the draft pull request
* ✅ Session is created
* Send several messages
* The chat nav is populated with the messages, ❓ but I can only select the first message
*
* I tried this in a few scenarios: I didn't have enough chat output to take up the full screen and for the first message to be out of view, and later when I sent more messages and the first message was out of view
* I found that I can only get certain messages to appear to be selected

Scenario 1: messages `hi` through `when was it created` are on the screen
* I can only get one of the first two messages (`hi` or `tell me more about the PR`) to appear selected
* The others will never appear to be selected in this session nav bar list (their background is never highlighted)
* If I select `who created the PR`, my screen correctly jumps to that message, but it never appears selected in the session nav bar list (its background is never highlighted)

Scenario 2: messages `tell me more about the PR` through `hi` are on the screen
* Same results as scenario 1