Windows Desktop: concurrent reasoning summaries correlate with ResizeObserver loops and UI freezes

Summary

On Codex/ChatGPT Desktop for Windows, the UI repeatedly became sluggish or temporarily unresponsive. The desktop log showed bursts of ResizeObserver loop completed with undelivered notifications errors. These bursts correlated almost exactly with concurrent reasoning-summary items being added or completed in the renderer.

Disabling concurrent_reasoning_summaries in the Codex config and fully restarting all ChatGPT/Codex processes stopped the visible freezing in this environment. No new ResizeObserver errors or Windows application hangs have been recorded since the targeted restart so far.

Related general Windows performance report: #20214.

Environment

• Codex/ChatGPT Desktop package: OpenAI.Codex_26.715.4045.0_x64
• Chromium/file version: 150.0.7871.124
• Windows 11 Pro 25H2, build 26200.8875, x64
• GPU: NVIDIA GeForce RTX 4090 plus AMD integrated graphics
• Two displays, 150% (device-scale-factor=1.5)
• Installed from the Microsoft Store/MSIX package

Observed behavior

• UI pauses and occasional Windows Not Responding state
• Delayed interaction while reasoning/status content updates
• 55 occurrences of the following renderer error in one approximately 13-minute app session:

[electron-message-handler] [desktop-notifications][global-error] ResizeObserver loop completed with undelivered notifications.

• Windows Event Viewer recorded three Application Hang events for ChatGPT.exe version 150.0.7871.124 during the affected period.
• An earlier app version (26.707.8479.0) also produced ChatGPT.exe crashes with exception code 0xc06d007f, although those older crashes may be a separate failure mode.

Correlation

The ResizeObserver errors repeatedly appeared immediately before or after log entries such as:

Reasoning summary item completed ... summaryPartCount=...
Reasoning summary part added ...

The app also logged:

Concurrent reasoning summaries feature override resolved featureOverride=true

Workaround

Add the following to %USERPROFILE%\.codex\config.toml:

[features]
concurrent_reasoning_summaries = false

Then fully terminate all ChatGPT.exe, codex.exe, and related Codex background processes before restarting the desktop app. Merely closing and reopening the window can leave the original renderer/background processes active.

The bundled Codex CLI confirms:

concurrent_reasoning_summaries    under development    false

After the full restart, the user reports that the freezing appears resolved. The post-restart observation currently shows zero new ResizeObserver errors and zero new Windows application hangs.

Expected behavior

Updating or completing reasoning-summary UI items should not trigger ResizeObserver loops or make the renderer unresponsive. If the experiment causes a layout feedback loop on Windows, it should be disabled for affected builds or the relevant resize callback should avoid synchronously changing observed dimensions.

Notes

• A clean profile and reinstall had not resolved the original freezing.
• The issue occurred before old conversations were restored, so restored history was not the initiating cause.
• GPU-disabled testing did not resolve the broader freezing, so hardware acceleration does not appear to be the sole cause in this environment.