Windows: semi-transparent sidebar causes transparent/undrawn left and top regions when maximized

Summary

On Codex Desktop for Windows, enabling Appearance -> Semi-transparent sidebar causes the maximized window to render incorrectly. The left sidebar area and the top/titlebar area become transparent or unpainted, so the desktop/background behind the window shows through. Disabling Semi-transparent sidebar immediately fixes the issue.

Steps to reproduce

1. Open Codex Desktop on Windows.
2. Go to Settings -> Appearance.
3. Enable Semi-transparent sidebar.
4. Optional but makes the bug easier to see: switch the theme to Dark.
5. Maximize the Codex window using any of these:
   • the maximize button,
   • Win + Up,
   • double-clicking the titlebar.
6. Observe the left and top areas of the window.
7. Disable Semi-transparent sidebar.
8. Maximize again.

Expected behavior

The Codex window should paint the full app/window area when maximized. If the sidebar is translucent, the translucency should remain clipped inside the app window and should not leave the sidebar or titlebar area transparent/unpainted.

Actual behavior

When Semi-transparent sidebar is enabled and the window is maximized, the left sidebar area and top/titlebar area are transparent or not drawn correctly. The desktop or white background behind the app shows through. In dark theme this is especially visible: the app content is dark, but the left/top regions appear missing.

Turning Semi-transparent sidebar off immediately makes the maximized window render normally.

Workaround

Disable Settings -> Appearance -> Semi-transparent sidebar.

Environment

• OS: Windows 10 22H2, build 19045
• GPU: AMD Radeon RX 6750 XT
• AMD Software: Adrenalin Edition 26.5.2
• GPU driver observed in Codex diagnostics/logs: 32.0.21043.10005
• CPU: AMD Ryzen 7 5700X
• RAM: 16 GB
• Display: 2560x1440
• Codex Desktop package observed: OpenAI.Codex_26.527.3686.0_x64__2p2nqsd0c76g0
• Codex app/log version observed: 26.527.31326

Troubleshooting already tried

• Reinstalled Codex Desktop.
• Reset Codex user profile/cache directories:
  • %APPDATA%\Codex
  • %LOCALAPPDATA%\Codex
• Cleaned GPU/Shader cache under the Codex profile.
• Tried disabling Chromium/Electron hardware acceleration via Local State; this made the issue worse, so it was reverted.
• Disabled Windows MPO using HKLM\SOFTWARE\Microsoft\Windows\Dwm\OverlayTestMode=5 and rebooted; no improvement.
• Verified the reliable workaround is specifically disabling Semi-transparent sidebar.

Notes

This looks like a Codex Desktop window composition/clipping bug around the semi-transparent sidebar on Windows. It does not appear to be fixed by reinstalling Codex, resetting the profile, disabling MPO, or changing GPU driver state.

Screenshots are available showing:

1. Semi-transparent sidebar enabled: maximized Codex has missing/transparent left and top areas.
2. Semi-transparent sidebar disabled: maximized Codex renders normally.