Many C++ module filetypes don't get syntax highlighting

What version of Codex CLI is running?

codex-cli 0.139.0

What subscription do you have?

Pro

Which model were you using?

gpt-5.5

What platform is your computer?

Linux 7.0.11-1-cachyos-server x86_64 unknown

What terminal emulator and version are you using (if applicable)?

foot version: 1.26.1 +pgo +ime +graphemes +toplevel-tag +blur -assertions

Codex doctor report

What issue are you seeing?

When codex proposes a diff to *.cxx or *.cpp files, the diffs are rendered nicely with syntax highlighting.
When it proposes diffs to *.cxxm or *.ixx files, they don't get syntax highlighting. Removals are red, additions are green. These don't seem to be recognized as C++, even though *.ixx is MSVC's standard for module interface units.

What steps can reproduce the bug?

Ask codex to create or modify a file with the ixx or cxxm extension.

What is the expected behavior?

Diffs to these files to be rendered in the same way as diffs to *.cpp or *.cxx files.

An alternative solution would be to have something like .gitattributes, where you can add *.ixx linguist-language=C++ to specify how extensions are interpreted.

Additional information

_No response_