Nova already previews Markdown, but its renderer is static HTML and CSS — there is no way to run a diagram library inside it. This extension takes the other route: it renders your Markdown into an ordinary HTML page inside the workspace and lets Nova preview that, which means anything a web page can do, the preview can do.

Using it

Open a Markdown file and choose Extensions → Mermaid Preview → Preview Markdown (⌃⇧M), or right-click in the editor.

The generated page opens as a tab. Switch it to Preview once — Editor → Preview, or the preview button in the tab bar — and Nova remembers that choice for the file. From then on the tab stays a preview and refreshes itself as the extension rewrites the page underneath it.

Split the two tabs side by side for the VS Code arrangement: source on the left, preview on the right.

.mmd and .mermaid files are previewed as a single diagram.

What gets rendered

CommonMark plus tables, strikethrough and autolinking, and:

```mermaid blocks, drawn as diagrams. Each one renders on its own, so a diagram that fails to parse shows its error and its source in place instead of blanking the document.
Task lists, as real checkboxes.
Headings, with ids, so a table of contents in the document links correctly.
Images and links relative to the Markdown file, rewritten so they still resolve from wherever the generated page lives.
YAML front matter, hidden.
Inline HTML, as written.

Settings

Setting	Default
Update preview	On save	On save, as you type, or only when asked.
Remove the generated page when its tab closes	On	Closing the preview, or the Markdown file, deletes the page. The shared assets go with the last one.
Theme	Auto	Auto follows the system appearance. Diagrams follow the same choice.
Content width	860	Maximum width of the text column, in pixels. Diagrams and tables may exceed it.
Generated files	`.nova/MermaidPreview`	Where generated pages and assets are written, relative to the preview root.

Extensions → Mermaid Preview → Remove Generated Files deletes the lot.

How it works, and what that costs

Nova has no API for contributing a previewer — an extension cannot supply the contents of a preview tab. What it can do is write a file and open it, and Nova's HTML preview is a real WebView that runs JavaScript. So:

The command writes <preview root>/.nova/MermaidPreview/<name>-<hash>.html, containing your Markdown as data plus a few script tags.
Alongside it, once per project, go markdown-it and mermaid and this extension's own stylesheet and page script (about 3.7 MB, copied only when missing or out of date).
Nova previews that page. Rendering happens in the WebView, where Mermaid has the DOM it needs to measure text and lay a diagram out.
Saving rewrites step 1 only — a couple of kilobytes — and the preview reloads.
Closing either tab deletes the page, and closing the last preview takes the assets and the directory with it.

The consequences worth knowing about:

Generated files live in your project. They have to: Nova's preview server only serves what is under the preview root. They are deleted again when the preview closes, but add .nova/MermaidPreview/ to .gitignore anyway — a crash or a force quit has no chance to tidy up.
The preview is a second tab, not an alternate view of the Markdown file. Nova's own Markdown preview still exists and is unchanged.
No scroll sync. There is no way to reach into the preview WebView from an extension. Scroll position does survive a reload.
Unsaved and remote files are not previewed — there is no path to resolve their images and links against.

Installing

Run ./install.sh with Nova quit; it copies the extension into Nova's user extension directory and registers it. During development, Extensions → Activate Project as Extension is quicker.

Credits

Bundles markdown-it and Mermaid, both MIT licensed.