--- a/layout/base/nsPresShell.cpp
+++ b/layout/base/nsPresShell.cpp
@@ -6867,18 +6867,26 @@ PresShell::HandleEvent(nsIFrame* aFrame,
       }
     }
   }
 
   if (sPointerEventEnabled) {
     UpdateActivePointerState(aEvent);
   }
 
-  if (!nsContentUtils::IsSafeToRunScript())
+  if (!nsContentUtils::IsSafeToRunScript() &&
+      aEvent->IsAllowedToDispatchDOMEvent()) {
+#ifdef DEBUG
+    if (aEvent->IsIMERelatedEvent()) {
+      nsPrintfCString warning("%d event is discarded", aEvent->message);
+      NS_WARNING(warning.get());
+    }
+#endif
     return NS_OK;
+  }
 
   nsIContent* capturingContent =
     (aEvent->HasMouseEventMessage() ||
      aEvent->eventStructType == NS_WHEEL_EVENT ? GetCapturingContent() :
                                                  nullptr);
 
   nsCOMPtr<nsIDocument> retargetEventDoc;
   if (!aDontRetargetEvents) {
@@ -7716,16 +7724,18 @@ PresShell::HandleEventInternal(WidgetEve
     // 2. Give event to the DOM for third party and JS use.
     if (NS_SUCCEEDED(rv)) {
       bool wasHandlingKeyBoardEvent =
         nsContentUtils::IsHandlingKeyBoardEvent();
       if (aEvent->eventStructType == NS_KEY_EVENT) {
         nsContentUtils::SetIsHandlingKeyBoardEvent(true);
       }
       if (aEvent->IsAllowedToDispatchDOMEvent()) {
+        MOZ_ASSERT(nsContentUtils::IsSafeToRunScript(),
+          "Somebody changed aEvent to cause a DOM event!");
         nsPresShellEventCB eventCB(this);
         if (aEvent->eventStructType == NS_TOUCH_EVENT) {
           DispatchTouchEvent(aEvent, aStatus, &eventCB, touchIsNew);
         } else {
           nsCOMPtr<nsINode> eventTarget = mCurrentEventContent.get();
           nsPresShellEventCB* eventCBPtr = &eventCB;
           if (!eventTarget) {
             nsCOMPtr<nsIContent> targetContent;