High CPU usage when returning to a chat that edited a file; persists after VS Code restart

What version of the Codex App are you using (From “About Codex” dialog)?

26.409.20454

What subscription do you have?

Plus

What platform is your computer?

uname -mprs

What issue are you seeing?

OS: Ubuntu 25.10
Editor: VS Code
CPU: Ryzen 9 9950X
Workspace: local C/C++ project
Symptom severity: about 4 of 16 cores become busy, causing noticeable heat and fan noise

The agent added a single inert comment line to a header file.
The line was later removed.
CPU usage stayed high even after the file was restored.
Restarting VS Code helped once earlier, but after reopening and re-activating the same chat, the high CPU usage returned.
Closing all editor tabs/windows did not help.
Observed behavior:

High CPU load is tied to the chat/session being active, not to the file contents.
In VS Code Process Explorer, Zygote is around 6% and Extension Host around 1%.
This suggests the load may be in a renderer/webview-related process rather than mainly in the extension host.

What steps can reproduce the bug?

• Open a chat.
• Ask the agent to make a small edit to a file.
• Keep that chat active or switch back to it later.
• CPU usage rises significantly.
• Leave that chat, and CPU usage drops.
• Return to the same chat, and CPU usage rises again.

What is the expected behavior?

Returning to a chat that previously edited a file should not cause sustained CPU load.
Reverting the edit should not leave the chat in a high-CPU state.

Additional information

The problem is reproducible with a trivial one-line edit.
The issue appears to persist with that specific chat thread/session.