Fix outstanding frontend_path issues

[masenf]

Summary

Fixes outstanding issues with the frontend_path config option and adds comprehensive integration tests to prevent regressions.

When frontend_path is set (e.g. frontend_path="/app"), the Reflex app is served from a subpath instead of the root. This PR ensures that assets, stylesheets, links, redirects, uploads, on_load events, and 404 pages all work correctly under a configured frontend_path.

Changes

• Integration tests (tests/integration/tests_playwright/test_frontend_path.py): Comprehensive playwright tests covering links (static and dynamic routes), redirects (event handler and on_load), assets (local and shared), uploaded files (rx.get_upload_url), CSS url() references, on_load event firing, and 404 pages. Tests are parametrized over dev/prod mode and with/without frontend_path.
• Shared asset fix (reflex/assets.py): rx.asset(shared=True) now calls prepend_frontend_path on its return value, matching the existing behavior for local assets.
• testing.py: Fix AppHarnessProd to locate 404.html under the frontend_path subdirectory when set, matching real prod deployment behavior.
• CI workflow (.github/workflows/integration_app_harness.yml): Run playwright tests in a separate job from other integration tests. The pytest-playwright plugin keeps an asyncio event loop running on the main thread for the session, which is incompatible with pytest-asyncio tests in the same process.

Closes

• Closes Add regression/integration tests for FRONTEND_PATH behaviour #5837
• Closes Base path configuration issue in subpath deployments (Knative/Kubernetes) #5597
• Closes [BUG] Infinite redirect loop when set frontend_path in rxconfig.py #5130
• Closes FRONTEND_PATH is not applied for assets URLs #4458

Supersedes

• Add integration tests for frontend path #5712 — The integration tests from that PR are incorporated and expanded here.

Test plan

• uv run pytest tests/integration/tests_playwright/test_frontend_path.py -v passes in both dev and prod modes, with and without frontend_path
• Existing integration tests still pass: uv run pytest tests/integration --ignore=tests/integration/tests_playwright -v
• Playwright tests are isolated from pytest-asyncio tests in CI

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

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_{Comparing masenf/fx-frontend-path (12cfd39) with main (4c53848)}

[greptile-apps]

Greptile Summary

This PR fixes frontend_path subpath deployment by replacing the Vite assetsDir hack with the proper base config option, centralising path-prefix logic in a new Config.prepend_frontend_path() helper, and wiring it into rx.asset (local), the Starlette static-file mount, the React Router basename, and the AppHarnessProd test harness.

• P1 — shared assets still broken: rx.asset(shared=True) returns an unprefixed URL (line 132 of reflex/assets.py). With frontend_path set, these URLs will 404 because the Starlette mount only handles paths under /prefix/.

Confidence Score: 4/5

Safe to merge for apps using local assets only; shared assets (rx.asset shared=True) will break in subpath deployments until the one-line fix is applied.

One P1 gap remains: shared/library assets are not prefixed, making them silently broken when frontend_path is configured. All other changes are clean refactors with correct equivalent behaviour and good integration-test coverage.

reflex/assets.py — the shared-asset return path (line 132) needs prepend_frontend_path.

Important Files Changed

Filename	Overview
reflex/assets.py	Fixes local assets with frontend_path, but shared assets (shared=True) still return an unprefixed URL at line 132 — they will 404 when frontend_path is set.
packages/reflex-base/src/reflex_base/config.py	Adds prepend_frontend_path() helper that consistently builds prefixed paths; logic is correct and all call-sites use it properly.
packages/reflex-base/src/reflex_base/compiler/templates.py	Switches Vite config from the assetsDir hack to the proper top-level base option, which is the correct way to handle subpath deployment.
reflex/utils/exec.py	Static files mount now points at the correct subdirectory (static/prefix/) and uses prepend_frontend_path for the mount path; looks correct.
reflex/testing.py	AppHarnessProd now derives the correct 404 page path and frontend_url from prepend_frontend_path, fixing prod-mode test harness for subpath apps.
tests/integration/tests_playwright/test_frontend_path.py	Comprehensive new integration tests for dev/prod × with/without prefix; CSS background-image assertion is too weak to catch a known Vite absolute-URL limitation.
reflex/app.py	Adds a vite config recompilation step during compilation (and increments the progress step counter to 8) so runtime config changes are reflected immediately.
reflex/utils/build.py	Only adds a clarifying comment; the actual file-moving logic is unchanged.
tests/units/test_prerequisites.py	Unit test expectations updated from assetsDir to base to match the new Vite config format.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A["Config.frontend_path = '/prefix'"] --> B["Config.prepend_frontend_path('/') → '/prefix/'"]
    B --> C["vite.config.js: base: '/prefix/'"]
    B --> D["react-router.config.js: basename: '/prefix/'"]
    B --> E["Starlette Mount('/prefix/')"]
    B --> F["rx.asset(local) → '/prefix/image.png'"]
    E --> G["StaticFiles(dir=static/prefix/)"]
    C --> H["Vite build: rewrites JS/CSS asset refs"]
    H --> I["build.py: mv static/* → static/prefix/"]
    I --> G
    J["rx.asset(shared=True)"] --> K["'/external/mod/img.png' ← NO prefix ❌"]
    K --> L["Browser requests /external/mod/img.png"]
    L --> M["Starlette 404 — not under /prefix/"]

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Comments Outside Diff (1)

1. reflex/assets.py, line 132 (link)

   Shared assets still missing frontend_path prefix

   rx.asset(shared=True) returns a URL without the frontend_path prefix. With frontend_path = "prefix" the Starlette static mount serves from static/prefix/ at /prefix/, so a browser request to /external/module/image.png will always 404 — it never matches the /prefix/ mount. Only local assets are fixed by this PR.

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