--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -8280,16 +8280,20 @@ PresShell::HandleEventInternal(WidgetEve
       if (sLastInputProcessed) {
         // This input event was created after we handled the last one.
         // Accumulate the previous events' coalesced duration.
         double lastMillis = (sLastInputProcessed - sLastInputCreated).ToMilliseconds();
         Telemetry::Accumulate(Telemetry::INPUT_EVENT_RESPONSE_COALESCED_MS,
                               lastMillis);
       }
       sLastInputCreated = aEvent->mTimeStamp;
+    } else if (aEvent->mTimeStamp < sLastInputCreated) {
+      // This event was created before the last input. May be processing out
+      // of order, so coalesce backwards, too.
+      sLastInputCreated = aEvent->mTimeStamp;
     }
     sLastInputProcessed = now;
   }
 
   return rv;
 }
 
 /* static */ void