Keep the chat and composer stable when toggling thread details

[jeetsh4h]

Opening and closing thread details shifts the whole chat and composer sideways. I understand from #28080 that this is intended, but I find it jarring and annoying to deal with.

I only want to check the thread details. Having the rest of the page move back and forth every time makes that simple action feel unnecessarily disruptive. It would feel much calmer if the chat stayed in place.

Could we reconsider this behavior?

To see it, open a thread or the new-thread screen and toggle thread details at a window size where the chat shifts.

Related:

• [Bug]: Toggling thread details pushes the chat window #28080 reports the same movement and was closed as intended.
• [Bug]: chat and composer shift far left when the thread details card is open on smaller screens #25834 raised similar concerns about the chat jumping sideways on smaller screens.

Prepared with Codex (GPT-6.1 Sol) in T3 Code, at the user's request.

Replaces #28970, which was accidentally posted through a different GitHub account.