Add preview run mode

[adhami3310]

What

Adds a new run mode: reflex run --env preview.

It hot reloads like dev, but instead of running the Vite dev server it serves a freshly built, un-minified frontend bundle mounted into the backend on a single port. Each backend hot reload re-runs the frontend build against the newly compiled output; a manual browser refresh shows changes.

The motivation: get dev-like iteration without the overhead of an always-running Vite server, while serving output much closer to a real production bundle (useful for reproducing "works in dev, breaks in prod" issues).

Note: unlike vite preview (which serves a static build with no rebuild), this mode still hot-reloads and rebuilds on each change.

How

• New Env.PREVIEW value, surfaced through run --env (and excluded from export --env, which stays dev/prod).
• _run_preview() reuses the existing prod static-mount path on a dev/reload backend — single process, no Vite.
• On each hot reload, App.__call__ re-runs build.build() (gated on preview mode + the frontend being mounted), wrapped so a build failure keeps serving the previous build instead of taking the backend down.

To keep cold rebuilds fast and the output debuggable, preview flips a few defaults (all overridable via env vars):

Setting	preview	Mechanism
JS + CSS minify	off	VITE_MINIFY → build.minify / build.cssMinify
autoprefixer	off	REFLEX_NO_AUTOPREFIXER, read by the generated postcss.config.js
sourcemaps	off (default)	VITE_SOURCEMAP

These came out of profiling: on a normal app the cold build is ~1.8s, dominated by toolchain startup, PostCSS/autoprefixer over the stylesheet, and react-router's SPA index.html step. The flips above shave ~0.3s (~15%) and land preview around ~1.5s.

Notes

• The postcss.config.js autoprefixer toggle keeps the autoprefixer: entry on a single line so the tailwind v3/v4 config rewriters' line-based parsing stays valid; postcss-load-config disables a plugin whose value is false. Existing apps pick this up on their next reinit.
• vite build --watch was evaluated as a faster alternative but rejected: it doesn't generate the SPA index.html and reintroduces a persistent process, defeating the goal.

Tests

• Env round-trip (incl. preview), is_prod_mode() stays False in preview.
• vite config emits minify/cssMinify from VITE_MINIFY.
• ruff, pyright, and unit tests pass.

Follow-ups (not in this PR)

• Docs entry for --env preview.

[FarhanAliRaza]

We can maybe call it preview. Like other frontend frameworks.

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 8 skipped benchmarks¹

---

_{Comparing dev-build-mode (ff68e20) with main (932f20f)}

Footnotes

1. 8 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[greptile-apps]

Greptile Summary

This PR adds reflex run --env preview, a new run mode that hot-reloads like dev but serves a freshly built, un-minified static frontend bundle mounted into the backend (single port, no Vite dev server). All three issues flagged in earlier review rounds — the autoprefixer truthiness bug, the unhandled build failure on hot reload, and the missing telemetry distinction — are addressed in the current HEAD.

• _run_preview() mirrors _run_prod() but calls exec.run_backend() (with Granian hot-reload) instead of exec.run_backend_prod(), and sets env defaults (VITE_MINIFY=false, REFLEX_NO_AUTOPREFIXER=true) before spawning workers so they are inherited correctly.
• On each hot reload, App.__call__ calls build.build() (gated on trigger == "hot_reload" + preview mode + frontend mounted) inside a try/except (Exception, SystemExit) so a failed build keeps the previous bundle alive.
• export is restricted to DEV/PROD; the run CLI now passes Env(env) through unchanged instead of collapsing everything non-DEV to PROD.

Confidence Score: 5/5

Safe to merge — the new code path is cleanly isolated, all three previously flagged issues are fixed, and the existing dev/prod paths are unchanged.

The env-var propagation model is correct (vars are set in the main process before Granian spawns workers), the .nocompile file mechanism prevents double-compilation on first startup, and the hot-reload rebuild is guarded and failure-tolerant. No regressions to existing modes were introduced.

No files require special attention.

Important Files Changed

Filename	Overview
reflex/reflex.py	Adds _run_preview(), routes Env.PREVIEW through _run(), fixes the run CLI to pass env values through unchanged, and restricts export to DEV/PROD only. Logic is correct: env defaults are set before the backend starts, so workers inherit them; the single-port constraint is applied consistently with prod.
reflex/app.py	Captures the compile trigger before _compile() and uses it to gate a preview-mode frontend rebuild on each hot reload; the build is wrapped in try/except so a failed build keeps serving the previous bundle. The REFLEX_MOUNT_FRONTEND_COMPILED_APP check ensures the rebuild only runs in fullstack preview, not backend-only or frontend-only.
packages/reflex-base/src/reflex_base/environment.py	Adds VITE_MINIFY (default True) and REFLEX_NO_AUTOPREFIXER (default False) env vars without internal=True, so their names are consistent between Python and the Node.js processes that read them.
packages/reflex-base/src/reflex_base/.templates/web/postcss.config.js	Adds a noVendorPrefix flag parsed with a case-insensitive regex so that REFLEX_NO_AUTOPREFIXER=false/0/empty correctly keeps autoprefixer enabled. The autoprefixer: entry stays on a single line to keep the tailwind v3/v4 config rewriters working.
packages/reflex-base/src/reflex_base/compiler/templates.py	Adds minify and cssMinify to the vite build config template, correctly emitting "true" or "false" based on the minify parameter; both JS and CSS minification are tied to the same flag.
tests/units/utils/test_utils.py	Adds four well-scoped tests: preview mode is not prod, Env enum round-trips correctly, the vite config template emits the right minify/cssMinify values, and _compile_vite_config reads VITE_MINIFY from the environment.

_{Reviews (4): Last reviewed commit: "preview: drop internal=True on REFLEX_NO..." | Re-trigger Greptile}

[adhami3310]

Good call — renamed the mode to preview (reflex run --env preview) in 4a391dd to match the Vite/Next/Nuxt convention. One nuance worth flagging for docs: unlike vite preview (which just serves a static build), this mode still hot-reloads and rebuilds on every change.