macOS: Translucent sidebar becomes opaque at window width >= 1920 px

Environment

• Codex App version: 26.820.60940
• Subscription: ChatGPT subscription (Pro)
• Platform: Darwin 25.6.0 arm64 arm (macOS 26)

What issue are you seeing?

With Settings -> Appearance -> Translucent sidebar enabled, the sidebar is translucent on the MacBook's built-in display but becomes opaque gray on an external display when the window width is 1920px or wider.

The external display is connected directly via USB-C. It remains the main display, and the behavior is the same with the MacBook lid open or closed. HDR is disabled, and changing display scaling does not change the behavior.

Steps to reproduce

1. Enable Settings -> Appearance -> Translucent sidebar.
2. Use a MacBook with a built-in display and an external USB-C display.
3. Move the Codex window between the two displays.
4. On the external display, resize the window and observe the sidebar at different widths.

The behavior has a deterministic breakpoint:

• window width 1919 px or less: sidebar is translucent;
• window width 1920 px or more: sidebar is opaque gray.

The built-in Retina display normally uses a logical width below 1920 px, while the external display reaches the 1920 px breakpoint. Resizing the window back to 1919 px immediately restores translucency.

Expected behavior

The Translucent sidebar setting should remain effective regardless of whether the window is on the built-in or external display, and regardless of the window width.

Additional information

This appears to be an unintended responsive breakpoint or wide-window fallback in the desktop renderer. The exact 1919/1920 px transition makes the behavior reproducible without relying on display hardware or focus state, however external display is needed as on the built-in the sidebar is translucent even when 1920px or wider.

Reproduce the issue from Settings -> Appearance -> Translucent sidebar on an external display, resizing the window across the 1919/1920 px breakpoint. Start by locating the desktop renderer's wide-window or responsive sidebar handling; done means the sidebar remains translucent at 1920 px and wider as well as on the built-in display.