Minimal reproduction: editing any MDX file that uses <Tabs> from @astrojs/starlight/components puts the dev server into a broken state. Vite triggers a program reload, then errors with HTMLElement is not defined while evaluating Starlight's Tabs.astro — specifically the client <script> block at line 146 (class StarlightTabs extends HTMLElement). The dev server stays in this state until restarted.

• astro@6.2.2
• @astrojs/starlight@0.38.5
• @astrojs/cloudflare@13.3.1
• @astrojs/mdx@5.0.4
• vite@7.3.1 (transitive)
• Node 22.14, macOS, run with bun@1.3.13 (also reproduces with npm/pnpm)

bun install
bun run dev

Open http://localhost:4321/ once so the page enters the module graph. Then edit src/content/docs/index.mdx (any change — add a trailing space to the title, save). The dev server log will print:

[watch] src/content/docs/index.mdx
[vite] program reload
[vite] An error happened during full reload
HTMLElement is not defined
ReferenceError: HTMLElement is not defined
    at eval (.../node_modules/@astrojs/starlight/user-components/Tabs.astro:146:42)
    at ESModulesEvaluator.runInlinedModule (.../vite/dist/node/module-runner.js:913:161)
    ...
    at async eval (.../node_modules/@astrojs/starlight/components.ts:6:1)

The page in the browser stops responding to further edits until the dev server is restarted.

@astrojs/starlight/components.ts is a barrel that re-exports every user component, including Tabs.astro. Any MDX importing from this barrel (e.g. just import { LinkCard } from '@astrojs/starlight/components') pulls the entire barrel — and therefore Tabs.astro — into the module graph.

On MDX edit Vite issues a program reload rather than letting Astro's HMR handle the change incrementally. During that reload Vite's SSR module runner re-evaluates Tabs.astro as a Node module, including the contents of its client <script> block. The script references HTMLElement, which doesn't exist in Node, and the reload aborts.

A first cold start works fine — <script> is correctly extracted for the client bundle. The problem is only on subsequent reloads.

• prerenderEnvironment: 'node' on the Cloudflare adapter is set in this repro because without it the page can't render in dev at all (workerd lacks Node compat for some Starlight transitive deps). Whether the bug itself depends on that flag is unclear — in a real project that has nodejs_compat in wrangler.jsonc, the same crash occurs without the flag.
• The bug does not depend on Svelte, custom Vite plugins, Starlight component overrides, output: 'server' vs output: 'static', or rehype-external-links — all bisected away in a real project before arriving at this minimum.
• Triggers on edits to any MDX in the docs collection that imports from @astrojs/starlight/components (even via the barrel for an unrelated component), as long as Tabs is somewhere in the graph.