using same default base

[jycouet]

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Latest commit: bf6191c

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[benmccann]

Is there an issue this fixes? We should probably also have a test for this. The base path stuff has been incredibly difficult to make work correctly

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-14866-svelte.vercel.app/

[jycouet]

It's actually the first part of #14070 and sveltejs/svelte#16663 (and part 2 only withastro/astro#12611)

Part 1: Vite config options will be overridden

cd packages/kit/test/apps/basic
pnpm test:unit unit-test/client.spec.js

Before this PR:

After this PR:

This log is happening before vitest launches, so I'm not sure how to test this.

I admit that I don't really know why in config_env.command === 'serve' we were setting base to kit.paths.base without any fallback, and causes this log by default.

Part 2: transport was disconnected, cannot call "fetchModule"

I think it's a vitest thing, but didn't manage to find yet the root cause (present in v3 and v4)
Let's see later for this one, it's not linked with this PR.