Mermaid in markdown breaks editor

### Short summary

When there is a longer mermaid entry in a markdown document, the markdown preview does no longer allow editing.

### Affected version or release

_No response_

### Installation context

_No response_

### What happened?

When working with Markdown files that include Mermaid diagrams, the editor switches to the canvas/preview experience. At that point, the file becomes effectively uneditable — the editor blocks and I can no longer make changes.
The preview/canvas view renders, but the editing experience breaks. I have to switch to code mode (or “agent/go-to-code mode”) to regain the ability to edit the file.

### Steps to reproduce

Open a Markdown file
Add or include Mermaid code block (e.g. ```mermaid)
Let the editor render/open the canvas/preview experience
Try to edit the file in the editor

### Expected behavior

I can continue editing the Markdown file normally
Preview/canvas view does not block editing
Switching between edit and preview works seamlessly

### Additional context

_No response_

より長い Mermaid コードブロックを含む Markdown ファイルを開き、キャンバスまたはプレビューの表示が開くのを待ってから編集を試み、問題を再現します。編集、プレビュー、コードモード間の遷移を追跡します。完了条件は、Markdown が編集可能なままで、ビューを切り替えても変更がブロックされなくなることです。

索引モデルが issue の本文から書いたものです。