--- a/layout/painting/FrameLayerBuilder.cpp
+++ b/layout/painting/FrameLayerBuilder.cpp
@@ -3592,16 +3592,20 @@ PaintedLayerData::AccumulateEventRegions
       !mVerticalPanRegion.IsEmpty();
   mNoActionRegion.OrWith(aEventRegions->NoActionRegion());
   mHorizontalPanRegion.OrWith(aEventRegions->HorizontalPanRegion());
   mVerticalPanRegion.OrWith(aEventRegions->VerticalPanRegion());
   if (alreadyHadRegions) {
     mDispatchToContentHitRegion.OrWith(CombinedTouchActionRegion());
   }
 
+  // Avoid quadratic performance as a result of the region growing to include
+  // and arbitrarily large number of rects, which can happen on some pages.
+  mMaybeHitRegion.SimplifyOutward(8);
+
   // Calculate scaled versions of the bounds of mHitRegion and mMaybeHitRegion
   // for quick access in FindPaintedLayerFor().
   mScaledHitRegionBounds = aState->ScaleToOutsidePixels(mHitRegion.GetBounds());
   mScaledMaybeHitRegionBounds = aState->ScaleToOutsidePixels(mMaybeHitRegion.GetBounds());
 }
 
 PaintedLayerData
 ContainerState::NewPaintedLayerData(nsDisplayItem* aItem,