diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index 045beb2..029b6ec 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,29 @@
+2020-05-22  Simon Fraser  <simon.fraser@apple.com>
+
+        Stuttery overflow scrolling in slow-scrolling regions (facebook messenger, feedly.com)
+        https://bugs.webkit.org/show_bug.cgi?id=212291
+        <rdar://problem/61940624>
+
+        Reviewed by Tim Horton.
+
+        Now that we do scrolling tree commits on the main thread, ThreadedScrollingTree::scrollingTreeNodeDidScroll()
+        can be called on the main thread. In this case, don't do an RunLoop::main().dispatch() which introduces
+        asynchrony; just call into the ScrollingCoordinator synchronously.
+
+        Do some minor refactoring to move noteScrollingThreadSyncCompleteForNode() into updateScrollPositionAfterAsyncScroll().
+
+        Hard to test because it involves scrolling thread/main thread interactions.
+
+        * page/scrolling/AsyncScrollingCoordinator.cpp:
+        (WebCore::AsyncScrollingCoordinator::requestScrollPositionUpdate):
+        (WebCore::AsyncScrollingCoordinator::synchronizeStateFromScrollingTree):
+        (WebCore::AsyncScrollingCoordinator::scheduleUpdateScrollPositionAfterAsyncScroll):
+        (WebCore::AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScrollTimerFired):
+        (WebCore::AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScroll):
+        * page/scrolling/AsyncScrollingCoordinator.h:
+        * page/scrolling/ThreadedScrollingTree.cpp:
+        (WebCore::ThreadedScrollingTree::scrollingTreeNodeDidScroll):
+
 2020-05-22  Zalan Bujtas  <zalan@apple.com>
 
         Nullptr deref in WebCore::RenderTreeBuilder::Block::attachIgnoringContinuation when parent and beforeChild are siblings
diff --git a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
index 008790c..e880ba0 100644
--- a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
+++ b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.cpp
@@ -264,7 +264,7 @@
     bool inBackForwardCache = frameView->frame().document()->backForwardCacheState() != Document::NotInBackForwardCache;
     bool inProgrammaticScroll = scrollableArea.currentScrollType() == ScrollType::Programmatic;
     if (inProgrammaticScroll || inBackForwardCache)
-        updateScrollPositionAfterAsyncScroll(scrollingNodeID, scrollPosition, { }, ScrollType::Programmatic, ScrollingLayerPositionAction::Set);
+        updateScrollPositionAfterAsyncScroll(scrollingNodeID, scrollPosition, { }, ScrollType::Programmatic, ScrollingLayerPositionAction::Set, InformWheelEventMonitor::No);
 
     ASSERT(inProgrammaticScroll == (scrollType == ScrollType::Programmatic));
 
@@ -299,7 +299,7 @@
     m_scrollingTree->traverseScrollingTree([&](ScrollingNodeID nodeID, ScrollingNodeType, Optional<FloatPoint> scrollPosition, Optional<FloatPoint> layoutViewportOrigin, bool scrolledSinceLastCommit) {
         if (scrollPosition && scrolledSinceLastCommit) {
             LOG_WITH_STREAM(Scrolling, stream << "AsyncScrollingCoordinator::synchronizeStateFromScrollingTree - node " << nodeID << " scroll position " << scrollPosition);
-            updateScrollPositionAfterAsyncScroll(nodeID, scrollPosition.value(), layoutViewportOrigin, ScrollType::User, ScrollingLayerPositionAction::Set);
+            updateScrollPositionAfterAsyncScroll(nodeID, scrollPosition.value(), layoutViewportOrigin, ScrollType::User, ScrollingLayerPositionAction::Set, InformWheelEventMonitor::No);
         }
     });
 }
@@ -331,10 +331,6 @@
         m_updateNodeScrollPositionTimer.stop();
         updateScrollPositionAfterAsyncScroll(m_scheduledScrollUpdate.nodeID, m_scheduledScrollUpdate.scrollPosition, m_scheduledScrollUpdate.layoutViewportOrigin, ScrollType::User, m_scheduledScrollUpdate.updateLayerPositionAction);
         updateScrollPositionAfterAsyncScroll(nodeID, scrollPosition, layoutViewportOrigin, ScrollType::User, scrollingLayerPositionAction);
-        
-        if (m_scheduledScrollUpdate.nodeID != nodeID)
-            noteScrollingThreadSyncCompleteForNode(m_scheduledScrollUpdate.nodeID);
-        noteScrollingThreadSyncCompleteForNode(nodeID);
         return;
     }
 
@@ -345,7 +341,6 @@
 void AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScrollTimerFired()
 {
     updateScrollPositionAfterAsyncScroll(m_scheduledScrollUpdate.nodeID, m_scheduledScrollUpdate.scrollPosition, m_scheduledScrollUpdate.layoutViewportOrigin, ScrollType::User, m_scheduledScrollUpdate.updateLayerPositionAction);
-    noteScrollingThreadSyncCompleteForNode(m_scheduledScrollUpdate.nodeID);
 }
 
 FrameView* AsyncScrollingCoordinator::frameViewForScrollingNode(ScrollingNodeID scrollingNodeID) const
@@ -380,10 +375,13 @@
     return nullptr;
 }
 
-void AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScroll(ScrollingNodeID scrollingNodeID, const FloatPoint& scrollPosition, Optional<FloatPoint> layoutViewportOrigin, ScrollType scrollType, ScrollingLayerPositionAction scrollingLayerPositionAction)
+void AsyncScrollingCoordinator::updateScrollPositionAfterAsyncScroll(ScrollingNodeID scrollingNodeID, const FloatPoint& scrollPosition, Optional<FloatPoint> layoutViewportOrigin, ScrollType scrollType, ScrollingLayerPositionAction scrollingLayerPositionAction, InformWheelEventMonitor informWheelEventMonitor)
 {
     ASSERT(isMainThread());
 
+    if (informWheelEventMonitor == InformWheelEventMonitor::Yes)
+        noteScrollingThreadSyncCompleteForNode(scrollingNodeID);
+
     if (!m_page)
         return;
 
diff --git a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.h b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.h
index 8a72041..09a74e5 100644
--- a/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.h
+++ b/Source/WebCore/page/scrolling/AsyncScrollingCoordinator.h
@@ -53,6 +53,9 @@
 
     WEBCORE_EXPORT void scheduleUpdateScrollPositionAfterAsyncScroll(ScrollingNodeID, const FloatPoint&, const Optional<FloatPoint>& layoutViewportOrigin, ScrollingLayerPositionAction);
 
+    enum class InformWheelEventMonitor { Yes, No };
+    void updateScrollPositionAfterAsyncScroll(ScrollingNodeID, const FloatPoint&, Optional<FloatPoint> layoutViewportOrigin, ScrollType, ScrollingLayerPositionAction, InformWheelEventMonitor = InformWheelEventMonitor::Yes);
+
 #if PLATFORM(COCOA)
     WEBCORE_EXPORT void handleWheelEventPhase(ScrollingNodeID, PlatformWheelEventPhase) final;
 
@@ -77,8 +80,6 @@
 
     RefPtr<ScrollingTree> releaseScrollingTree() { return WTFMove(m_scrollingTree); }
 
-    void updateScrollPositionAfterAsyncScroll(ScrollingNodeID, const FloatPoint&, Optional<FloatPoint> layoutViewportOrigin, ScrollType, ScrollingLayerPositionAction);
-
     WEBCORE_EXPORT String scrollingStateTreeAsText(ScrollingStateTreeAsTextBehavior = ScrollingStateTreeAsTextBehaviorNormal) const override;
     WEBCORE_EXPORT String scrollingTreeAsText(ScrollingStateTreeAsTextBehavior = ScrollingStateTreeAsTextBehaviorNormal) const override;
     WEBCORE_EXPORT void willCommitTree() override;
diff --git a/Source/WebCore/page/scrolling/ThreadedScrollingTree.cpp b/Source/WebCore/page/scrolling/ThreadedScrollingTree.cpp
index 6b267b0..93c8160 100644
--- a/Source/WebCore/page/scrolling/ThreadedScrollingTree.cpp
+++ b/Source/WebCore/page/scrolling/ThreadedScrollingTree.cpp
@@ -148,6 +148,11 @@
 
     LOG_WITH_STREAM(Scrolling, stream << "ThreadedScrollingTree::scrollingTreeNodeDidScroll " << node.scrollingNodeID() << " to " << scrollPosition << " bouncing to main thread");
 
+    if (RunLoop::isMain()) {
+        m_scrollingCoordinator->updateScrollPositionAfterAsyncScroll(node.scrollingNodeID(), scrollPosition, layoutViewportOrigin, ScrollType::User, scrollingLayerPositionAction);
+        return;
+    }
+
     RunLoop::main().dispatch([strongThis = makeRef(*this), nodeID = node.scrollingNodeID(), scrollPosition, layoutViewportOrigin, scrollingLayerPositionAction] {
         if (auto* scrollingCoordinator = strongThis->m_scrollingCoordinator.get())
             scrollingCoordinator->scheduleUpdateScrollPositionAfterAsyncScroll(nodeID, scrollPosition, layoutViewportOrigin, scrollingLayerPositionAction);