Markdown file becomes read-only in rendered mode when it contains a `` ```mermaid `` block

[teddyberco]

Description

In the Editor canvas, adding a mermaid fenced code block to a markdown file makes the entire file uneditable in rendered (WYSIWYG) mode — not just the diagram block. Text before and after the diagram, headings, and list items all reject input. Switching to <> source mode restores editing.

Steps to reproduce

1. Open a markdown file in the Editor canvas with this content:

Markdown

# Mermaid edit test

Paragraph BEFORE the diagram — try typing here.

```mermaid
flowchart LR
  A[User] --> B[Skill]
  B --> C[Fabric]

Paragraph AFTER the diagram — try typing here too.

• list item before
• list item after

1. Stay in rendered mode.

2. Click into "Paragraph BEFORE the diagram" and type.



**Expected:** Surrounding prose is editable; at most the rendered diagram itself is an atomic/non-editable node.


**Actual:** Nothing is editable anywhere in the file. Editing only works after switching to `<>` source mode.


**Control test (isolates the trigger)**

The same file with `` ```mermaid `` replaced by `` ```text `` is fully editable in rendered mode. The mermaid block is the trigger.


**Impact**

Markdown specs that embed architecture diagrams — a documented, encouraged pattern — lose WYSIWYG editing entirely. In our spec repo this affects 41 files.


**Environment**



- GitHub Copilot app (CLI) v1.0.80

- Windows

- Editor canvas, rendered mode