ensure TransitionRoot component without props transitions correctly

A bit of a weird one, but you can use the `TransitionRoot` component
without any props for transitions themselves (so no real transition can
happen). Even crazier, it can happen that it doesn't even render a DOM
node, but just its children.

At this point, the `TransitionRoot` component is purely there for
orchestration purposes of child components.

Since there is no DOM node in certain situations, the transitions (and
its `onStart` and `onStop` callbacks) won't even happen at all. This
causes a bug (obvious in react strict mode) where children don't
properly mount or the transition component doesn't properly unmount.

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

@@ -559,6 +559,7 @@ function TransitionRootFn<TTag extends ElementType = typeof DEFAULT_TRANSITION_C
   let [state, setState] = useState(show ? TreeStates.Visible : TreeStates.Hidden)
 
   let nestingBag = useNesting(() => {
+    if (show) return
     setState(TreeStates.Hidden)
   })
 
@@ -590,7 +591,7 @@ function TransitionRootFn<TTag extends ElementType = typeof DEFAULT_TRANSITION_C
   useIsoMorphicEffect(() => {
     if (show) {
       setState(TreeStates.Visible)
-    } else if (!hasChildren(nestingBag)) {
+    } else if (!hasChildren(nestingBag) && internalTransitionRef.current !== null) {
       setState(TreeStates.Hidden)
     }
   }, [show, nestingBag])

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

@@ -35,23 +35,31 @@ export function useTransition({ container, direction, classes, onStart, onStop }
   let inFlight = useRef(false)
 
   useIsoMorphicEffect(() => {
-    let node = container.current
-    if (!node) return // We don't have a DOM node (yet)
     if (direction === 'idle') return // We don't need to transition
     if (!mounted.current) return
 
     onStart.current(direction)
 
-    d.add(
-      transition(node, {
-        direction,
-        classes: classes.current,
-        inFlight,
-        done() {
-          onStop.current(direction)
-        },
-      })
-    )
+    let node = container.current
+    if (!node) {
+      // No node, so let's skip the transition and call the `onStop` callback
+      // immediately because there is no transition to wait for anyway.
+      onStop.current(direction)
+    }
+
+    // We do have a node, let's transition it!
+    else {
+      d.add(
+        transition(node, {
+          direction,
+          classes: classes.current,
+          inFlight,
+          done() {
+            onStop.current(direction)
+          },
+        })
+      )
+    }
 
     return d.dispose
   }, [direction])