Add integration tests for frontend path

[tartansandal]

Introduces an integration test for #5674.
This xfails on main but passes on #5698

Edit: also fixed an apparent bug in test_link_hover.py. There may be a deeper issue here.
Edit: removed xfail after merging main which now includes #5698.
Edit: added a test for 404s.

[codspeed-hq]

Merging this PR will not alter performance

✅ 8 untouched benchmarks

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_{Comparing tartansandal:kjh/integration-tests-for-frontend-path (0afccdf) with main (3c11451)}

[greptile-apps]

Greptile Summary

This PR introduces integration testing for frontend path functionality, specifically addressing issue #5674 where on_load and on_mount handlers fail to redirect properly when Reflex applications are served from non-root frontend paths (e.g., '/prefix').

The changes include two main components:

1. Environment Variable Addition: A new REFLEX_FRONTEND_PATH environment variable is added to the EnvironmentVariables class in reflex/environment.py. This string variable defaults to an empty string and provides the configuration mechanism for specifying the base path under which the frontend is served.

2. Comprehensive Integration Test: A new test file tests/integration/tests_playwright/test_frontend_path.py creates a complete end-to-end test using Playwright. The test creates a Reflex app with redirect pages and validates that redirects work correctly both in normal conditions and when served from a non-root frontend path.

The integration test uses two fixtures - one for production mode and another that sets the REFLEX_FRONTEND_PATH to '/prefix'. It employs parametrized testing to verify both scenarios, with the prefix case marked as xfail since it demonstrates the known bug that should be resolved by the related PR #5698.

This change fits into the broader Reflex ecosystem by enabling proper deployment behind reverse proxies, a common pattern where multiple applications are served from different paths on the same domain. The environment variable provides a clean configuration interface that other parts of the codebase can reference to adjust their behavior for non-root deployments.

Confidence score: 4/5

• This PR introduces well-structured integration testing with minimal risk to existing functionality
• Score reflects solid test design and environment variable implementation, though the xfail test indicates known issues
• Pay close attention to the test file to ensure proper cleanup of environment variables and test isolation

_{2 files reviewed, 2 comments}

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[tartansandal]

Note I'm using simple environment module tweaks to simulate changing the environment. An alternative would be to modify AppHarness to take a config object. That would be a much larger change, but might be useful testing some configuration scenarios.

[shahshahii]

Shah

[masenf]

needs to be updated to remove the new Environment variables (those are exposed by default), and have the test update the Config instance directly.

[tartansandal]

Thanks for taking another look at this! 🙏

I've rebased onto current main and revised the approach based on your feedback:

• Removed all environment.py changes — those env vars have become defaults since this PR was originally created, so the additions were redundant.
• Switched to os.environ for setting REFLEX_FRONTEND_PATH and REFLEX_SHOW_BUILT_WITH_REFLEX in the test fixtures. I initially tried mutating the Config instance directly (e.g. get_config().frontend_path = "/prefix"), but AppHarness._initialize_app calls get_config(reload=True) which creates a fresh Config from disk and wipes out any prior mutations. Setting the values via os.environ means they survive the reload through Config.update_from_env() — and it mirrors how users actually configure these in practice.
• Fixed AppHarnessProd._run_frontend — it was hardcoding the 404.html path at the web root, but when frontend_path is set the build output (including 404.html) is nested under that prefix. Updated it to match what sirv does in real prod deployments.
• Added prod-mode prefix tests — the original fixtures were inconsistently using AppHarnessProd for baselines but AppHarness (dev mode) for prefix cases. Now both dev and prod are covered for the prefix scenarios.

All 7 tests passing, pre-commit checks green ✅

[tartansandal]

TIL: monkeypatch.setenv 😄