fix!: only serve dev static assets from publish dir as fallback

[Copilot]

Fixes #436

Problem

When using @netlify/vite-plugin, vite dev (or e.g. astro dev) was incorrectly serving files from the Netlify publish dir (e.g. dist/) instead of the source directory during vite dev.

This causes various problems, the most basic of which is that if you've ever previously run vite build (or e.g. `astro build) your dev server scans the production build output dir and serves assets from it. This is confusing and incorrect.

Solution

I can't think of any reason to ever augment the directories explicitly passed to @netlify/dev's options with a default directory. In every known use case (@netlify/vite-plugin, @netlify/nuxt...), we pass the known, exhaustive set of directories that should be used to serve static assets in dev, so we should respect these as is.

It seems reasonable to use some sensible defaults here when not provided (for hypothetical future or direct use cases).

This PR makes this change.

Changes

• Updated implementation to use only custom directories when provided
• Added comprehensive tests
• Documented the default behavior with @default JSDoc tag

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[serhalp]

@copilot what does this have to do with SPAs and redirects? did you cross wires with another issue?

also, can you compare this with the behaviour implemented in https://github.com/netlify/cli/? when does it use static files in the publish dir during dev?

[Copilot]

@copilot what does this have to do with SPAs and redirects? did you cross wires with another issue?

also, can you compare this with the behaviour implemented in https://github.com/netlify/cli/? whe...

The SPA redirect is directly related to the issue. Here's how:

1. The reproduction has a SPA-style rewrite rule: /* -> /index.html (status 200)
2. When a request comes in, the redirect handler matches it and rewrites to /index.html
3. The redirect handler then looks for /index.html in the static directories
4. With the old code, static directories were [dist, projectRoot, publicDir], so dist/index.html was found first
5. The middleware sees result.type === 'redirect' (not 'static'), so it serves the response directly (line 117-120 in main.ts)
6. This causes the built dist/index.html to be served instead of the source index.html transformed by Vite

Without the SPA redirect, the issue would still exist but be less obvious - direct requests for files that exist in both dist and source would serve from dist.

Regarding comparison with Netlify CLI: The CLI doesn't integrate with Vite's dev server, so it doesn't have this conflict. During vite dev, we explicitly want to serve from source directories only (projectRoot + publicDir), not from the build output. The fix ensures that when vite-plugin provides custom directories, those are used exclusively. When @netlify/dev is used standalone (no custom directories), it still falls back to the original behavior of using build.publish.

Commit: 71ed852

[serhalp]

Note: this PR is ready as is. I just wanted to do some local testing to verify the fix (for #513 as well at the same time). I'll get to it eventually, so I'm keeping these open.

[serhalp]

@copilot document the default here

[Copilot]

Updated the documentation to use @default JSDoc tag and more precisely describe the fallback behavior: build.publish from netlify.toml if configured, otherwise projectRoot.

Commit: 23e4a66