diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index a554b0ab9183d..2cc14ba0831b4 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3360,6 +3360,22 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
     }
   }
 
+  bool isRootContent =
+      mIsRoot && mOuter->PresContext()->IsRootContentDocumentCrossProcess();
+
+  nsRect effectiveScrollPort = mScrollPort;
+  if (isRootContent && mOuter->PresContext()->HasDynamicToolbar()) {
+    // Expand the scroll port to the size including the area covered by dynamic
+    // toolbar in the case where the dynamic toolbar is being used since
+    // position:fixed elements attached to this root scroller might be taller
+    // than its scroll port (e.g 100vh). Even if the dynamic toolbar covers the
+    // taller area, it doesn't mean the area is clipped by the toolbar because
+    // the dynamic toolbar is laid out outside of our topmost window and it
+    // transitions without changing our topmost window size.
+    effectiveScrollPort.SizeTo(nsLayoutUtils::ExpandHeightForViewportUnits(
+        mOuter->PresContext(), effectiveScrollPort.Size()));
+  }
+
   // It's safe to get this value before the DecideScrollableLayer call below
   // because that call cannot create a displayport for root scroll frames,
   // and hence it cannot create an ignore scroll frame.
@@ -3376,8 +3392,8 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
   nsRect visibleRect = aBuilder->GetVisibleRect();
   nsRect dirtyRect = aBuilder->GetDirtyRect();
   if (!ignoringThisScrollFrame) {
-    visibleRect = visibleRect.Intersect(mScrollPort);
-    dirtyRect = dirtyRect.Intersect(mScrollPort);
+    visibleRect = visibleRect.Intersect(effectiveScrollPort);
+    dirtyRect = dirtyRect.Intersect(effectiveScrollPort);
   }
 
   bool dirtyRectHasBeenOverriden = false;
@@ -3495,7 +3511,7 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
                StyleOverflowClipBox::ContentBox;
     // We only clip if there is *scrollable* overflow, to avoid clipping
     // *visual* overflow unnecessarily.
-    nsRect clipRect = mScrollPort + aBuilder->ToReferenceFrame(mOuter);
+    nsRect clipRect = effectiveScrollPort + aBuilder->ToReferenceFrame(mOuter);
     nsRect so = mScrolledFrame->GetScrollableOverflowRect();
     if ((cbH && (clipRect.width != so.width || so.x < 0)) ||
         (cbV && (clipRect.height != so.height || so.y < 0))) {
@@ -3537,23 +3553,11 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
 
   nsDisplayListCollection set(aBuilder);
 
-  bool isRootContent =
-      mIsRoot && mOuter->PresContext()->IsRootContentDocumentCrossProcess();
   bool willBuildAsyncZoomContainer =
       aBuilder->ShouldBuildAsyncZoomContainer() && isRootContent;
 
-  nsRect scrollPortClip = mScrollPort + aBuilder->ToReferenceFrame(mOuter);
-  // Expand the clip rect to the size including the area covered by dynamic
-  // toolbar in the case where the dynamic toolbar is being used since
-  // position:fixed elements attached to this root scroller might be taller than
-  // its scroll port (e.g 100vh). Even if the dynamic toolbar covers the taller
-  // area, it doesn't mean the area is clipped by the toolbar because the
-  // dynamic toolbar is laid out outside of our topmost window and it
-  // transitions without changing our topmost window size.
-  if (isRootContent && mOuter->PresContext()->HasDynamicToolbar()) {
-    scrollPortClip.SizeTo(nsLayoutUtils::ExpandHeightForViewportUnits(
-        mOuter->PresContext(), scrollPortClip.Size()));
-  }
+  nsRect scrollPortClip =
+      effectiveScrollPort + aBuilder->ToReferenceFrame(mOuter);
   nsRect clipRect = scrollPortClip;
   // Our override of GetBorderRadii ensures we never have a radius at
   // the corners where we have a scrollbar.
@@ -3796,7 +3800,7 @@ void ScrollFrameHelper::BuildDisplayList(nsDisplayListBuilder* aBuilder,
       info += CompositorHitTestFlags::eRequiresTargetConfirmation;
     }
 
-    nsRect area = mScrollPort + aBuilder->ToReferenceFrame(mOuter);
+    nsRect area = effectiveScrollPort + aBuilder->ToReferenceFrame(mOuter);
 
     // Make sure that APZ will dispatch events back to content so we can
     // create a displayport for this frame. We'll add the item later on.