Bug 1365333 - Avoid quadratic performance in nsDisplayLayerEventRegions::AddFrame() when the maybe-hit region has many rects. r=tnikkel

MozReview-Commit-ID: 99QrhFpHw15

--HG--
extra : rebase_source : 07201ca9b9728e9b1fa77755614ce8cd95753430

layout/painting/nsDisplayList.cpp

@@ -4422,6 +4422,10 @@ nsDisplayLayerEventRegions::AddFrame(nsDisplayListBuilder* aBuilder,
   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);
   }