Opening one

await app.openResource('file:///notes/todo.md');                   // into `main`
await app.openResource(uri, { surface: 'aside', mode: 'edit' });

The caller never names a component.

The worked example

playground/src/examples/ is the filesystem explorer the brief asks for: a real provider over node:fs, kinds for text, markdown, code and data, viewers for each, a fallback for everything else, registered actions, and the JSON adapter on top. Read explorer.tsx and note what is not imported - there is no MarkdownViewer in it, no JsonViewer, and no if (ext === '.md').

It also shows the two rules that keep a viewer from wrecking a layout: every pane is a fixed size or a flex share, and the viewer scrolls inside its pane. Opening a four-line file and a four-thousand-line one produce the same frame - there is a test that asserts exactly that.

Run it:

pnpm dev explorer