Codex desktop chat renders fenced code blocks in a proportional font

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

26.818.61809 (7019), bundled codex-cli 0.149.0-alpha.4.3

What subscription do you have?

Not relevant / unknown

What platform is your computer?

macOS 26.6.2 (Build 25G83)

What issue are you seeing?

Fenced Markdown code blocks in Codex desktop chat render using a proportional UI font instead of a fixed-width/monospace code font.

The app recognizes the content as a code block (it displays the code-block container and copy button), but the glyph widths are proportional. This breaks column alignment in ASCII trees, tables, diagrams, and any whitespace-aligned plain text.

This occurs with both an untyped fenced block and a fenced text block.

What steps can reproduce the bug?

1. Open a Codex desktop task.
2. Ask Codex to return this inside a fenced Markdown block, first without a language and then using ```text:

LLM runtime
    │
    └── Agent harness
        ├── MCP → Search
        └── MCP → Browser

3. Compare the widths of narrow and wide glyphs, or inspect whether the tree connectors remain vertically aligned.
4. Observe that the block uses proportional typography and its columns do not align.

What is the expected behavior?

All fenced code blocks, including plain-text blocks, should use the configured Code font and render with fixed-width glyphs so whitespace alignment is preserved.

Additional information

The same content was re-rendered using both untyped and text fences with the same result. The code-block chrome appears correctly, so this looks like a font-family inheritance or code-font application problem in the chat renderer rather than Markdown parsing.

A screenshot demonstrating the issue is available from the originating Codex task but could not be attached through the issue API.

Start by inspecting the Codex desktop chat renderer's fenced-code styling and how it applies the configured Code font. Reproduce the issue with both untyped and text fences, then verify that narrow and wide glyphs align and whitespace-based trees, tables, and diagrams remain fixed-width.