Thinking header not anchored — expanding a collapsed Thinking entry scrolls it out of view

Testing #325512

When expanding a **collapsed** Thinking entry in chat, the entry is **not anchored** — instead of staying pinned in place, it scrolls away entirely, so the header I clicked is no longer in view and I lose my place.

Expected (per the TPI): the Thinking header should stay **anchored** and not move with the rest of the chat when expanded.

## Steps to reproduce
1. Set `chat.agent.thinkingStyle` to `collapsed`.
2. Send a prompt that produces reasoning + tool calls (a long Thinking block).
3. Once the Thinking entry is collapsed, click to **expand** it.
4. ❌ The entry scrolls out of view / the header is not pinned, rather than staying anchored in place.