Queue discrete events in microtask

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

@@ -716,21 +716,27 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
-      root.callbackNode = null;
-      root.callbackPriority = NoLanePriority;
     }
+    root.callbackNode = null;
+    root.callbackPriority = NoLanePriority;
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  if (existingCallbackNode !== null) {
-    const existingCallbackPriority = root.callbackPriority;
-    if (existingCallbackPriority === newCallbackPriority) {
-      // The priority hasn't changed. We can reuse the existing task. Exit.
-      return;
+  const existingCallbackPriority = root.callbackPriority;
+  if (existingCallbackPriority === newCallbackPriority) {
+    if (__DEV__) {
+      invariant(
+        existingCallbackNode,
+        'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
+      );
     }
-    // The priority changed. Cancel the existing callback. We'll schedule a new
-    // one below.
+    // The priority hasn't changed. We can reuse the existing task. Exit.
+    return;
+  }
+
+  if (existingCallbackNode != null) {
+    // Cancel the existing callback. We'll schedule a new one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -739,6 +745,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
+
+    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
     newCallbackNode = scheduleSyncCallback(
       performSyncWorkOnRoot.bind(null, root),
     );
@@ -1879,6 +1887,7 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
+  root.callbackPriority = NoLanePriority;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

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

@@ -698,21 +698,27 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     // Special case: There's nothing to work on.
     if (existingCallbackNode !== null) {
       cancelCallback(existingCallbackNode);
-      root.callbackNode = null;
-      root.callbackPriority = NoLanePriority;
     }
+    root.callbackNode = null;
+    root.callbackPriority = NoLanePriority;
     return;
   }
 
   // Check if there's an existing task. We may be able to reuse it.
-  if (existingCallbackNode !== null) {
-    const existingCallbackPriority = root.callbackPriority;
-    if (existingCallbackPriority === newCallbackPriority) {
-      // The priority hasn't changed. We can reuse the existing task. Exit.
-      return;
+  const existingCallbackPriority = root.callbackPriority;
+  if (existingCallbackPriority === newCallbackPriority) {
+    if (__DEV__) {
+      invariant(
+        existingCallbackNode,
+        'Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue.',
+      );
     }
-    // The priority changed. Cancel the existing callback. We'll schedule a new
-    // one below.
+    // The priority hasn't changed. We can reuse the existing task. Exit.
+    return;
+  }
+
+  if (existingCallbackNode != null) {
+    // Cancel the existing callback. We'll schedule a new one below.
     cancelCallback(existingCallbackNode);
   }
 
@@ -721,6 +727,8 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
   if (newCallbackPriority === SyncLanePriority) {
     // Special case: Sync React callbacks are scheduled on a special
     // internal queue
+
+    // TODO: After enableDiscreteEventMicroTasks lands, we can remove the fake node.
     newCallbackNode = scheduleSyncCallback(
       performSyncWorkOnRoot.bind(null, root),
     );
@@ -1859,6 +1867,7 @@ function commitRootImpl(root, renderPriorityLevel) {
   // commitRoot never returns a continuation; it always finishes synchronously.
   // So we can clear these now to allow a new callback to be scheduled.
   root.callbackNode = null;
+  root.callbackPriority = NoLanePriority;
 
   // Update the first and last pending times on this root. The new first
   // pending time is whatever is left on the root fiber.

scripts/error-codes/codes.json

@@ -372,5 +372,6 @@
   "381": "This feature is not supported by ReactSuspenseTestUtils.",
   "382": "This query has received more parameters than the last time the same query was used. Always pass the exact number of parameters that the query needs.",
   "383": "This query has received fewer parameters than the last time the same query was used. Always pass the exact number of parameters that the query needs.",
-  "384": "Refreshing the cache is not supported in Server Components."
+  "384": "Refreshing the cache is not supported in Server Components.",
+  "385": "Expected scheduled callback to exist. This error is likely caused by a bug in React. Please file an issue."
 }