Bug 1527516 - Recompute mIsScrollableLayerInRootContainer correctly if containerless scrolling is disabled during the scroll frame's lifetime. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D19603

--HG--
extra : moz-landing-system : lando

layout/generic/nsGfxScrollFrame.cpp

@@ -3893,10 +3893,9 @@ bool ScrollFrameHelper::DecideScrollableLayer(
     aBuilder->RecomputeCurrentAnimatedGeometryRoot();
   }
 
-  if (gfxPrefs::LayoutUseContainersForRootFrames() &&
-      mWillBuildScrollableLayer && mIsRoot) {
-    mIsScrollableLayerInRootContainer = true;
-  }
+  mIsScrollableLayerInRootContainer =
+      gfxPrefs::LayoutUseContainersForRootFrames() &&
+      mWillBuildScrollableLayer && mIsRoot;
   return mWillBuildScrollableLayer;
 }