Fix double beforeEnter due to SSR (#1183)

* prevent initial transitioning in SSR environment

Due to SSR and the hydration step, the transition code was already
called even if we were not ready yet. This caused an issue where the
`beforeEnter` callback got fired twice intead of once.

Fixes: #311

* update changelog

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Improve outside click support ([#1175](https://github.com/tailwindlabs/headlessui/pull/1175))
 - Ensure that `appear` works regardless of multiple rerenders ([#1179](https://github.com/tailwindlabs/headlessui/pull/1179))
 - Reset Combobox Input when the value gets reset ([#1181](https://github.com/tailwindlabs/headlessui/pull/1181))
+- Fix double `beforeEnter` due to SSR ([#1183](https://github.com/tailwindlabs/headlessui/pull/1183))
 
 ## [Unreleased - @headlessui/vue]
 

packages/@headlessui-react/src/components/transitions/transition.tsx

@@ -279,6 +279,7 @@ let TransitionChild = forwardRefWithAs(function TransitionChild<
   useIsoMorphicEffect(() => {
     let node = container.current
     if (!node) return
+    if (!ready) return
     if (skip) return
     if (show === prevShow.current) return
 
@@ -328,6 +329,7 @@ let TransitionChild = forwardRefWithAs(function TransitionChild<
     container,
     skip,
     show,
+    ready,
     enterClasses,
     enterFromClasses,
     enterToClasses,