Codex desktop: horizontal scrollbar closes expanded Mermaid diagrams

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

Windows 26.810.52044; macOS 26.810.41047

What subscription do you have?

Windows Plus; macOS Enterprise

What platform is your computer?

Windows desktop and macOS desktop. The issue reproduces on both platforms. Exact OS build and architecture were not captured.

What issue are you seeing?

The expanded Mermaid diagram viewer closes when I use the horizontal scrollbar to scroll to the right.

The diagram renders inline, and clicking “Expand Mermaid diagram” opens the expanded viewer. The failure occurs in both the Windows and macOS Codex desktop apps with the versions listed above. I reproduced it using a long left-to-right Mermaid flowchart. This appears to be an interaction bug in the expanded preview, not a Mermaid syntax error.

This is related to #32788 (expanded Mermaid preview bug) and #10842 (Mermaid full-screen/zoom request), but the specific behavior here is that horizontal scrolling closes the expanded viewer.

What steps can reproduce the bug?

1. Open the Codex desktop app.
2. Ask Codex to render a wide Mermaid diagram.
3. Use a long left-to-right flowchart such as the following:

flowchart LR
    A["Request received"] --> B["Authenticate user"]
    B --> C["Validate request"]
    C --> D["Load configuration"]
    D --> E["Check permissions"]
    E --> F["Select pipeline"]
    F --> G["Normalize input"]
    G --> H["Enrich metadata"]
    H --> I["Check duplicates"]
    I --> J["Queue background job"]
    J --> K["Allocate worker"]
    K --> L["Fetch records"]
    L --> M["Transform records"]
    M --> N["Run validation"]
    N --> O["Calculate results"]
    O --> P["Apply business rules"]
    P --> Q["Run quality checks"]
    Q --> R["Store results"]
    R --> S["Update index"]
    S --> T["Return response"]

4. Click “Expand Mermaid diagram.”
5. Drag the expanded diagram’s horizontal scrollbar to the right.
6. Observe that the expanded diagram closes.
7. Repeat on both Windows and macOS.

What is the expected behavior?

The expanded Mermaid viewer should remain open while the horizontal scrollbar moves the diagram to the right.

Additional information

Reproduced on:

• Windows Codex desktop app 26.810.52044, Plus plan
• macOS Codex desktop app 26.810.41047, Enterprise plan

The issue is reproducible with a wide left-to-right Mermaid diagram. Related reports: #32788 and #10842.

Start from the “Expand Mermaid diagram” entry point in the Codex desktop app and reproduce the issue with the provided wide left-to-right flowchart. Inspect the expanded viewer’s horizontal-scroll interaction; done means dragging the scrollbar right leaves the viewer open on both Windows and macOS.