Check if there's a partial tree before restarting

If a partial render expires, we should stay in the concurrent path
(performConcurrentWorkOnRoot); we'll stop yielding, but the rest of the
behavior remains the same.

We will only revert to the sync path (performSyncWorkOnRoot) when
starting on a new level.

This approach prevents partially completed concurrent work from
being discarded.

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -644,9 +644,16 @@ function performConcurrentWorkOnRoot(root, didTimeout) {
   // event time. The next update will compute a new event time.
   currentEventTime = NoWork;
 
-  if (didTimeout) {
-    // The render task took too long to complete. Mark the current time as
-    // expired to synchronously render all expired work in a single batch.
+  // Check if the render expired. If so, restart at the current time so that we
+  // can finish all the expired work in a single batch. However, we should only
+  // do this if we're starting a new tree. If we're in the middle of an existing
+  // tree, we'll continue working on that (without yielding) so that the work
+  // doesn't get dropped. If there's another expired level after that, we'll hit
+  // this path again, at which point we can batch all the subsequent levels
+  // together.
+  if (didTimeout && workInProgress === null) {
+    // Mark the current time as expired to synchronously render all expired work
+    // in a single batch.
     const currentTime = requestCurrentTimeForUpdate();
     markRootExpiredAtTime(root, currentTime);
     // This will schedule a synchronous callback.