diff --git a/dom/ipc/TabParent.cpp b/dom/ipc/TabParent.cpp
index 767eb03491a4..8184d415ddb3 100644
--- a/dom/ipc/TabParent.cpp
+++ b/dom/ipc/TabParent.cpp
@@ -1809,10 +1809,9 @@ TabParent::RecvOnEventNeedingAckHandled(const EventMessage& aMessage)
 {
   // This is called when the child process receives WidgetCompositionEvent or
   // WidgetSelectionEvent.
+  // FYI: Don't check if widget is nullptr here because it's more important to
+  //      notify mContentCahce of this than handling something in it.
   nsCOMPtr<nsIWidget> widget = GetWidget();
-  if (!widget) {
-    return IPC_OK();
-  }
 
   // While calling OnEventNeedingAckHandled(), TabParent *might* be destroyed
   // since it may send notifications to IME.
diff --git a/widget/ContentCache.cpp b/widget/ContentCache.cpp
index 87d5f45b796a..810e3b5b6233 100644
--- a/widget/ContentCache.cpp
+++ b/widget/ContentCache.cpp
@@ -1126,6 +1126,12 @@ ContentCacheInParent::OnCompositionEvent(const WidgetCompositionEvent& aEvent)
     MOZ_ASSERT(aEvent.mMessage == eCompositionChange ||
                aEvent.mMessage == eCompositionCommit);
     *mCommitStringByRequest = aEvent.mData;
+    // We need to wait eCompositionCommitRequestHandled from the remote process
+    // in this case.  Therefore, mPendingEventsNeedingAck needs to be
+    // incremented here.
+    if (!mWidgetHasComposition) {
+      mPendingEventsNeedingAck++;
+    }
     return false;
   }
 
@@ -1165,7 +1171,8 @@ ContentCacheInParent::OnEventNeedingAckHandled(nsIWidget* aWidget,
      "aMessage=%s), mPendingEventsNeedingAck=%u, mPendingCompositionCount=%" PRIu8,
      this, aWidget, ToChar(aMessage), mPendingEventsNeedingAck, mPendingCompositionCount));
 
-  if (WidgetCompositionEvent::IsFollowedByCompositionEnd(aMessage)) {
+  if (WidgetCompositionEvent::IsFollowedByCompositionEnd(aMessage) ||
+      aMessage == eCompositionCommitRequestHandled) {
     MOZ_RELEASE_ASSERT(mPendingCompositionCount > 0);
     mPendingCompositionCount--;
   }
diff --git a/widget/EventMessageList.h b/widget/EventMessageList.h
index c42a4514e2ed..c5358671ab85 100644
--- a/widget/EventMessageList.h
+++ b/widget/EventMessageList.h
@@ -198,6 +198,14 @@ NS_EVENT_MESSAGE(eCompositionCommitAsIs)
 // After that, eCompositionEnd will be dispatched automatically.
 // Its mRanges should be nullptr.
 NS_EVENT_MESSAGE(eCompositionCommit)
+// eCompositionCommitRequestHandled is NOT used with any Widget*Event.
+// This is used only by PBrowser.OnEventNeedingAckHandled().  If active IME
+// commits composition synchronously, TabParent returns the commit string
+// to the remote process synchronously.  Then, TabChild dispatches
+// eCompositionCommit in the remote process.  Finally, this message is sent
+// to TabParent.  (If IME commits composition asynchronously, this message is
+// not used.)
+NS_EVENT_MESSAGE(eCompositionCommitRequestHandled)
 
 // Following events are defined for deprecated DOM events which are using
 // InternalUIEvent class.
diff --git a/widget/PuppetWidget.cpp b/widget/PuppetWidget.cpp
index 643e6c2ca20d..edbc7b974b0d 100644
--- a/widget/PuppetWidget.cpp
+++ b/widget/PuppetWidget.cpp
@@ -653,6 +653,9 @@ PuppetWidget::RequestIMEToCommitComposition(bool aCancel)
   nsEventStatus status = nsEventStatus_eIgnore;
   DispatchEvent(&compositionCommitEvent, status);
 
+  Unused <<
+    mTabChild->SendOnEventNeedingAckHandled(eCompositionCommitRequestHandled);
+
   // NOTE: PuppetWidget might be destroyed already.
   return NS_OK;
 }