fix transition enter bug (#1519)

We had an issue where an open Dialog got hidden by css didn't properly
unmount because the Transition never "finished". We fixed this by
checking if the node was hidden by using `getBoundingClientRect`.

Today I learned that just *reading* those values (aka call
`node.getBoundingClientRect()`) it for whatever reason completely stops
the transition. This causes the enter transitions to completely stop
working.

Instead, we move this code so that we only check the existence of the
Node when we try to transition out because this means that the Node is
definitely there, just have to check its bounding rect.

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

@@ -339,7 +339,8 @@ let TransitionRoot = forwardRefWithAs(function Transition<
 >(props: TransitionChildProps<TTag> & { show?: boolean; appear?: boolean }, ref: Ref<HTMLElement>) {
   // @ts-expect-error
   let { show, appear = false, unmount, ...theirProps } = props as typeof props
-  let transitionRef = useSyncRefs(ref)
+  let internalTransitionRef = useRef<HTMLElement | null>(null)
+  let transitionRef = useSyncRefs(internalTransitionRef, ref)
 
   // The TransitionChild will also call this hook, and we have to make sure that we are ready.
   useServerHandoffComplete()
@@ -390,6 +391,15 @@ let TransitionRoot = forwardRefWithAs(function Transition<
       setState(TreeStates.Visible)
     } else if (!hasChildren(nestingBag)) {
       setState(TreeStates.Hidden)
+    } else {
+      let node = internalTransitionRef.current
+      if (!node) return
+      let rect = node.getBoundingClientRect()
+
+      if (rect.x === 0 && rect.y === 0 && rect.width === 0 && rect.height === 0) {
+        // The node is completely hidden, let's hide it
+        setState(TreeStates.Hidden)
+      }
     }
   }, [show, nestingBag])
 

packages/@headlessui-react/src/hooks/use-transition.ts

@@ -72,17 +72,6 @@ export function useTransition({
     if (latestDirection.current === 'idle') return // We don't need to transition
     if (!mounted.current) return
 
-    dd.add(() => {
-      if (!node) return
-
-      let rect = node.getBoundingClientRect()
-
-      if (rect.x === 0 && rect.y === 0 && rect.width === 0 && rect.height === 0) {
-        // The node is completely hidden
-        onStop.current(latestDirection.current)
-      }
-    })
-
     dd.dispose()
 
     beforeEvent()