diff --git a/packages/react-dom/src/events/plugins/__tests__/SimpleEventPlugin-test.js b/packages/react-dom/src/events/plugins/__tests__/SimpleEventPlugin-test.js
index 377fa61e507d7..795eda90c83cc 100644
--- a/packages/react-dom/src/events/plugins/__tests__/SimpleEventPlugin-test.js
+++ b/packages/react-dom/src/events/plugins/__tests__/SimpleEventPlugin-test.js
@@ -477,8 +477,8 @@ describe('SimpleEventPlugin', function() {
         // At the end, both counters should equal the total number of clicks
         expect(Scheduler).toHaveYielded([
           'High-pri count: 8, Low-pri count: 0',
-
-          // TODO: with cancellation, this required another flush?
+        ]);
+        expect(Scheduler).toFlushAndYield([
           'High-pri count: 8, Low-pri count: 8',
         ]);
       } else {
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
index 6935bb4b1f18d..918261a2e3562 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.new.js
@@ -1028,6 +1028,20 @@ function performSyncWorkOnRoot(root) {
     exitStatus = renderRootSync(root, lanes);
   } else {
     lanes = getNextLanes(root, NoLanes);
+    // Because we don't cancel synchronous tasks, sometimes more than one
+    // synchronous task ends up being scheduled. This is an artifact of the fact
+    // that we have two different lanes that schedule sync tasks: discrete and
+    // sync. If we had only one, then (I believe) this extra check wouldn't be
+    // necessary, because there's nothing higher priority than sync that would
+    // cause us to cancel it.
+    // TODO: Merge InputDiscreteLanePriority with SyncLanePriority, then delete
+    // this bailout.
+    if (supportsMicrotasks) {
+      const nextLanesPriority = returnNextLanesPriority();
+      if (nextLanesPriority < InputDiscreteLanePriority) {
+        return null;
+      }
+    }
     exitStatus = renderRootSync(root, lanes);
   }
 
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
index 2b57becf8bd79..12ef384a47527 100644
--- a/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
+++ b/packages/react-reconciler/src/ReactFiberWorkLoop.old.js
@@ -1028,6 +1028,20 @@ function performSyncWorkOnRoot(root) {
     exitStatus = renderRootSync(root, lanes);
   } else {
     lanes = getNextLanes(root, NoLanes);
+    // Because we don't cancel synchronous tasks, sometimes more than one
+    // synchronous task ends up being scheduled. This is an artifact of the fact
+    // that we have two different lanes that schedule sync tasks: discrete and
+    // sync. If we had only one, then (I believe) this extra check wouldn't be
+    // necessary, because there's nothing higher priority than sync that would
+    // cause us to cancel it.
+    // TODO: Merge InputDiscreteLanePriority with SyncLanePriority, then delete
+    // this bailout.
+    if (supportsMicrotasks) {
+      const nextLanesPriority = returnNextLanesPriority();
+      if (nextLanesPriority < InputDiscreteLanePriority) {
+        return null;
+      }
+    }
     exitStatus = renderRootSync(root, lanes);
   }