Visual Studio Code preview doesn't render Mermaid diagrams in markdown files

I don't get Mermaid diagrams contained in a markdown file rendered in the Visual Studio Code default previewer; instead, the code block is displayed verbatim; the Mermaid Chart extension is installed; example content of a markdown file:

# Title
This is some text

```mermaid
---
config:
  layout: elk
---

flowchart TB
      Start[sdf afd]@{ shape: stadium}
      VarDef[Is Variable defined?]
```