Long threads can jump back to top while scrolling downward toward latest output

Bug Report

App version: 26.406.31014 (1395)
Platform: macOS

Description

In very long conversations, scrolling downward toward the latest output can suddenly reset the viewport back to the top of the thread.

This makes it very difficult to get back to the newest messages in long chats.

This may be related to #17169, but the user-visible symptom here is specifically:

• not just scrolling up causing a jump to the center
• but also scrolling downward in long threads causing the view to jump back to the top

Steps to Reproduce

1. Open Codex app on macOS (version 26.406.31014 / build 1395)
2. Open a conversation with a very long history
3. Start from somewhere above the latest messages
4. Scroll downward toward the newest output

Expected

The thread should keep its current scroll position and allow smooth navigation toward the latest output.

Actual

The viewport can unexpectedly jump back to the top of the conversation, forcing the user to scroll down again.

Impact

High. In long threads, returning to the latest output becomes very time-consuming and frustrating.

Additional Context

This started after updating to 26.406.31014 (1395).