Bundle postcss in standalone SSR builds

[dawoodkhan82]

Description

Vite 8 externalizes postcss from the SSR output, but the Node build shipped in the wheel does not install it. This makes style sanitization fail at runtime and every SSR render return HTTP 500.

This PR bundles postcss through ssr.noExternal and adds a build-time check that copies the generated server and production dependencies into an isolated temporary directory, then performs an SSR render. Future accidentally externalized runtime dependencies will therefore fail the frontend build.

Closes: #13672

AI Disclosure

• I used AI to investigate the regression, implement the fix and standalone build check, and draft this PR. I self-reviewed every changed line.
• I did not use AI

🎯 PRs Should Target Issues

Issue #13672 describes the regression and reproduction. No overlapping open issue or PR was found before opening this PR.

Testing and Formatting Your Code

• pnpm --filter @self/app build
• node js/app/verify_build.js
• node --test js/app/proxy_routes.test.js (60 passed)
• Local Gradio SSR demo: Node listens on port 7860 and curl http://127.0.0.1:7860/ returns HTTP 200
• bash scripts/format_frontend.sh formatted the patch; its repository-wide type-check step still reports existing errors in unrelated frontend files

Minimal demo (kept untracked):

import gradio as gr

with gr.Blocks() as demo:
    gr.HTML('<div style="color: red">SSR standalone build works</div>')

if __name__ == "__main__":
    demo.launch(ssr_mode=True, server_port=7860, debug=True)

[gradio-pr-bot]

🪼 branch checks and previews

•	Name	Status	URL
	Spaces	ready!	Spaces preview
	Storybook	ready!	Storybook preview
🦄	Changes	detected!	Details

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Install Gradio from this PR

pip install https://huggingface.co/buckets/gradio/pypi-previews/resolve/2399b03260a1971df98b119d854b67053c9f386f/gradio-6.20.0-py3-none-any.whl

Install Gradio Python Client from this PR

pip install "gradio-client @ git+https://github.com/gradio-app/gradio@2399b03260a1971df98b119d854b67053c9f386f#subdirectory=client/python"

Import Gradio JS Client from this PR via CDN

import { Client } from "https://huggingface.co/buckets/gradio/npm-previews/resolve/2399b03260a1971df98b119d854b67053c9f386f/browser.js";

[gradio-pr-bot]

🦄 change detected

This Pull Request includes changes to the following packages.

Package	Version
@self/app	minor

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• Bundle postcss in standalone SSR builds

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Something isn't right?

• Maintainers can change the version label to modify the version bump.
• If the bot has failed to detect any changes, or if this pull request needs to update multiple packages to different versions or requires a more comprehensive changelog entry, maintainers can update the changelog file directly.

[Copilot]

Pull request overview

This PR fixes a regression in Gradio’s standalone SSR Node build (shipped in the Python wheel) where postcss is externalized in Vite 8 SSR output and therefore missing at runtime, causing SSR renders to fail. It also adds a build-time verification step that copies the built SSR output into an isolated temp directory and performs an SSR render to catch future accidentally-externalized runtime dependencies during the frontend build.

Changes:

• Bundle postcss into the SSR output via ssr.noExternal in the Vite config.
• Add js/app/verify_build.js to perform an isolated SSR render verification as part of the build pipeline.
• Run the verification step during @self/app’s build script.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
js/app/vite.config.ts	Adds postcss to SSR noExternal to ensure it’s bundled into the SSR output.
js/app/verify_build.js	New build-time isolated SSR render verification script.
js/app/package.json	Runs verify_build.js as part of the build script.
.changeset/thin-donuts-make.md	Adds a changeset entry for @self/app.

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[abidlabs]

Great LGTM and confirmed https://huggingface.co/spaces/gradio-pr-deploys/pr-13673-all-demos is up now