Check root.hydrate before scheduling root to be cleared

This avoids breaking support for ReactDOM.hydration in legacy mode with no fallback.

Author: Brian Vaughn

packages/react-reconciler/src/ReactFiberCompleteWork.new.js

@@ -681,11 +681,13 @@ function completeWork(
           // If we hydrated, then we'll need to schedule an update for
           // the commit side-effects on the root.
           markUpdate(workInProgress);
-        } else {
+        } else if (!fiberRoot.hydrate) {
           // Schedule an effect to clear this container at the start of the next commit.
           // This handles the case of React rendering into a container with previous children.
           // It's also safe to do for updates too, because current.child would only be null
           // if the previous render was null (so the the container would already be empty).
+          //
+          // The additional root.hydrate check is required for hydration in legacy mode with no fallback.
           workInProgress.effectTag |= Snapshot;
         }
       }

packages/react-reconciler/src/ReactFiberCompleteWork.old.js

@@ -679,11 +679,13 @@ function completeWork(
           // If we hydrated, then we'll need to schedule an update for
           // the commit side-effects on the root.
           markUpdate(workInProgress);
-        } else {
+        } else if (!fiberRoot.hydrate) {
           // Schedule an effect to clear this container at the start of the next commit.
           // This handles the case of React rendering into a container with previous children.
           // It's also safe to do for updates too, because current.child would only be null
           // if the previous render was null (so the the container would already be empty).
+          //
+          // The additional root.hydrate check is required for hydration in legacy mode with no fallback.
           workInProgress.effectTag |= Snapshot;
         }
       }