Clear finished discrete updates during commit phase

If a root is finished at a priority lower than that of the latest pending discrete
updates on it, these updates must have been finished so we can clear them now.
Otherwise, a later call of `flushDiscreteUpdates` would start a new empty render
pass which may cause a scheduled timeout to be cancelled.

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -1769,6 +1769,16 @@ function commitRootImpl(root, renderPriorityLevel) {
     remainingExpirationTimeBeforeCommit,
   );
 
+  // Clear already finished discrete updates in case that a later call of
+  // `flushDiscreteUpdates` starts a useless render pass which may cancels
+  // a scheduled timeout.
+  if (rootsWithPendingDiscreteUpdates !== null) {
+    const lastDiscreteTime = rootsWithPendingDiscreteUpdates.get(root);
+    if (lastDiscreteTime !== undefined && lastDiscreteTime >= expirationTime) {
+      rootsWithPendingDiscreteUpdates.delete(root);
+    }
+  }
+
   if (root === workInProgressRoot) {
     // We can reset these now that they are finished.
     workInProgressRoot = null;