[P0 / Blocker][Windows App] Assistant responses are visually clipped and only readable via Copy last message

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

Codex App 26.616.6631.0; bundled Codex CLI/app-server 0.142.0-alpha.6.

What subscription do you have?

ChatGPT subscription (exact tier not relevant to the rendering issue).

What platform is your computer?

Windows 10 x64, version 22H2, build 19045.6466.

What issue are you seeing?

Severity: P0 / Blocker

Assistant responses are visually clipped or missing in the Codex Windows app. The conversation viewport can show only a lower portion of an assistant response, while the beginning and other content are not visible or reachable through normal scrolling.

The response data itself is intact: using Copy last message copies the complete response to the clipboard. Therefore this appears to be a renderer, virtualized-list height, or scroll-position/layout calculation bug rather than a model or transport truncation issue.

This happens repeatedly and makes the app unusable without copying every response into another application.

Impact:

• Important commands, warnings, and context are hidden.
• Users may execute incomplete deployment commands or YAML because the visible response is only a fragment.
• Every response must be copied to the clipboard and read elsewhere.
• The issue persists whether Codex is launched/used from a VS Code workflow or directly from PowerShell.

What steps can reproduce the bug?

1. Run the Codex Windows app on Windows 10.
2. Open a long-running thread.
3. Ask Codex for a response containing multiple paragraphs and a YAML or shell code block.
4. Wait for the response to finish.
5. Observe that the message viewport shows only part of the assistant response; earlier content is clipped or cannot be reached normally.
6. Use Copy last message and paste it into a text editor.
7. Observe that the clipboard contains the full response, including content that the Codex UI did not render visibly.
8. Repeat with Codex launched from a VS Code workflow and from PowerShell; the behavior persists.

Frequency: persistent / repeatedly reproducible.

What is the expected behavior?

The complete assistant response should render in the conversation and remain reachable through normal scrolling, regardless of message length, code blocks, window size, or launch method.

Additional information

• Screenshot captured on 2026-07-16 shows the visible response beginning in the middle of a YAML block, with the earlier part unavailable, while the app reports “Copied last message to clipboard.”
• This is not the same as #25693. In #25693, copied file-viewer content is truncated. Here, clipboard content is complete, but the conversation renderer clips the displayed assistant message.
• Local application logs identify the installed Windows app as OpenAI.Codex_26.616.6631.0_x64 and show the renderer running in the packaged Electron app.
• No model/transport error is observed; the full message is available through the clipboard.

Requested labels: bug, app, windows-os.