regression: mentions v2 (`@` command) the selected candidate is hard to distinguish in TUI (bad UI experience)

Summary

In Codex TUI v0.140.0, the mentions v2 (@) candidate list is much harder to scan than the older mentions v1 UI because the current selection is only bolded while keeping the same foreground/background color as the surrounding candidates.

In the older mentions v1 UI, the currently selected candidate was easy to identify because it was both bolded and highlighted with a different color. With mentions v2, the selected candidate still uses the same color treatment as the other candidates, so the only visual signal is text weight. That makes the current selection difficult to find quickly.

User impact

This is especially noticeable when using an external monitor at a normal working distance. From farther away, the selected candidate can become hard to distinguish because humans may not reliably perceive the difference between regular and bold text at that distance. The candidate can look distinguishable in a cropped local screen recording, but the real monitor experience is worse than the video suggests.

Comparison recordings

I have two short recordings that show the difference.

https://github.com/user-attachments/assets/ba138494-cc61-4488-a4e3-68cf70e0a16f

https://github.com/user-attachments/assets/71ebcf14-957a-41b6-9c5a-bf7db20ad2be

• mentions-v1-very-easy-to-find-candidates.mov: older mentions v1 behavior, where the selected candidate is both bolded and highlighted with a different color.
• mentions-v2-low-contrast-hard-to-find-current-selected-candidates.mov: v0.140.0 mentions v2 behavior, where the selected candidate is only bolded and keeps the same color, making it low contrast and hard to find.

Expected behavior

The currently selected mention candidate should have a clear visual highlight, similar to mentions v1. Ideally the UI should restore the v1-style treatment where the active candidate is not only bolded but also highlighted with a distinct color/background.

Actual behavior

The active candidate in mentions v2 appears to be differentiated only by bold text. The lack of a distinct color/background highlight makes the selection state hard to read and easy to lose track of.

Environment

• Codex version: v0.140.0
• UI: TUI
• Feature: mentions v2 (@) candidate selection