fix(build): use correct base for chunk import map

[vaibhavmashal]

Fixes #23350. The issue drops module preload dependency lists when base is not /. This changes \getImportMapBaseUrl\ to return \options.base\ instead of /\ when \chunkImportMap.baseUrl\ is missing.

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[vaibhavmashal]

Here is a detailed explanation of what was done in this PR:

The Problem

When \�uild.chunkImportMap\ is enabled and \�ase\ is not /, the module preload dependency lists get dropped (__vite__mapDeps\ is absent). This happens because Vite injected one \�aseUrl\ into
olldown, but when reading it back, it fell back to /\ since the user's
olldownOptions\ (which it read) lacked the \�aseUrl. As a result, the preload markers didn't work and CSS wasn't requested for lazily imported chunks.

The Fix

In \packages/vite/src/node/plugins/html.ts, the function \getImportMapBaseUrl\ was updated to read from \options.base\ instead of falling back blindly to /.

\\ s
function getImportMapBaseUrl(
options: ResolvedEnvironmentOptions & Pick<ResolvedConfig, 'base'>,
): string {
const chunkImportMap =
options.build.rolldownOptions.experimental?.chunkImportMap
if (typeof chunkImportMap === 'object' && chunkImportMap.baseUrl) {
return chunkImportMap.baseUrl
}
return options.base ?? '/' // Now respects the base if present
}
\\

By changing this one line, __vite__mapDeps\ is preserved properly, the CSS gets requested, and the mapped key reverts to using the stable ID as expected.

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