--- a/layout/painting/nsDisplayList.cpp
+++ b/layout/painting/nsDisplayList.cpp
@@ -4417,16 +4417,20 @@ nsDisplayLayerEventRegions::AddFrame(nsD
     if (clip->GetRoundedRectCount() > 0) {
       borderBoxHasRoundedCorners = true;
     }
   }
 
   if (borderBoxHasRoundedCorners ||
       (aFrame->GetStateBits() & NS_FRAME_SVG_LAYOUT)) {
     mMaybeHitRegion.Or(mMaybeHitRegion, borderBox);
+
+    // Avoid quadratic performance as a result of the region growing to include
+    // an arbitrarily large number of rects, which can happen on some pages.
+    mMaybeHitRegion.SimplifyOutward(8);
   } else {
     mHitRegion.Or(mHitRegion, borderBox);
   }
 
   if (aBuilder->IsBuildingNonLayerizedScrollbar() ||
       aBuilder->GetAncestorHasApzAwareEventHandler())
   {
     // Scrollbars may be painted into a layer below the actual layer they will