diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index 6acc956028b5..201c639ccf20 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -2227,7 +2227,6 @@ ScrollFrameHelper::ScrollFrameHelper(nsContainerFrame* aOuter, bool aIsRoot)
       mHasBeenScrolledRecently(false),
       mWillBuildScrollableLayer(false),
       mIsParentToActiveScrollFrames(false),
-      mAddClipRectToLayer(false),
       mHasBeenScrolled(false),
       mIgnoreMomentumScroll(false),
       mTransformingByAPZ(false),
@@ -3699,10 +3698,6 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
   MOZ_ASSERT(sf);
 
   if (ignoringThisScrollFrame) {
-    // Root scrollframes have FrameMetrics and clipping on their container
-    // layers, so don't apply clipping again.
-    mAddClipRectToLayer = false;
-
     // If we are a root scroll frame that has a display port we want to add
     // scrollbars, they will be children of the scrollable layer, but they get
     // adjusted by the APZC automatically.
@@ -3740,11 +3735,6 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     return;
   }
 
-  // Root scrollframes have FrameMetrics and clipping on their container
-  // layers, so don't apply clipping again.
-  mAddClipRectToLayer =
-      !(mIsRoot && mOuter->PresShell()->UsesMobileViewportSizing());
-
   // Whether we might want to build a scrollable layer for this scroll frame
   // at some point in the future. This controls whether we add the information
   // to the layer tree (a scroll info layer if necessary, and add the right
diff --git a/layout/generic/nsGfxScrollFrame.h b/layout/generic/nsGfxScrollFrame.h
index ae69fdc869a3..722056912084 100644
--- a/layout/generic/nsGfxScrollFrame.h
+++ b/layout/generic/nsGfxScrollFrame.h
@@ -683,10 +683,6 @@ class ScrollFrameHelper : public nsIReflowCallback {
   // display ports removed or made minimal, then we expire our display port.
   bool mIsParentToActiveScrollFrames : 1;
 
-  // If true, add clipping in ScrollFrameHelper::ClipLayerToDisplayPort.
-  // XXX this flag needs some auditing and better documentation, bug 1646686.
-  bool mAddClipRectToLayer : 1;
-
   // True if this frame has been scrolled at least once
   bool mHasBeenScrolled : 1;