Does not behave as monospaced in GitHub file editor, markdown code blocks etc

We've all seen wonky ascii diagrams like this and assumed LLMs aren't counting the character columns correctly when generating them:

That's partly true. But when I went to edit the diagram by hand I found that I couldn't make the lines align.

In fact if I navigate the cursor up and down across the lines it takes a zig-zag path!

This is not supposed to happen with a monospaced font!

The issue appears to be the "Monaspace Neon" font that GitHub are using is not behaving as monospaced. After editing the CSS to get an actual monospaced font the chars align in true columns and I was able to edit the diagram into shape. But unfortunately after saving it still renders as per the screenshot at the top - yes, that's the 'fixed' version as rendered in Monaspace Neon.

See this video:

https://github.com/user-attachments/assets/e09cf31e-9ff2-4cb6-a349-6b193ea507b8

Non-fixed-width columns seems like a serious flaw for something supposed to be a monospace font?