diff --git a/layout/base/nsDisplayList.cpp b/layout/base/nsDisplayList.cpp
index f48387f106e..79ff5b003bf 100644
--- a/layout/base/nsDisplayList.cpp
+++ b/layout/base/nsDisplayList.cpp
@@ -622,83 +622,6 @@ static bool GetApzcTreePrintPref() {
   return gPrintApzcTree;
 }
 
-static CSSSize
-CalculateRootCompositionSize(FrameMetrics& aMetrics,
-                             bool aIsRootContentDocRootScrollFrame,
-                             nsPresContext* aPresContext,
-                             nsIFrame* aForFrame, nsIFrame* aScrollFrame)
-{
-
-  if (aIsRootContentDocRootScrollFrame) {
-    return ViewAs<LayerPixel>(ParentLayerSize(aMetrics.mCompositionBounds.Size()),
-                              PixelCastJustification::ParentLayerToLayerForRootComposition)
-           / aMetrics.LayersPixelsPerCSSPixel();
-  }
-  LayerSize rootCompositionSize;
-  nsPresContext* rootPresContext =
-    aPresContext->GetToplevelContentDocumentPresContext();
-  if (!rootPresContext) {
-    rootPresContext = aPresContext->GetRootPresContext();
-  }
-  nsIPresShell* rootPresShell = nullptr;
-  if (rootPresContext) {
-    // See the comments in the code that calculates the root
-    // composition bounds in RecordFrameMetrics.
-    // TODO: Reuse that code here.
-    nsIPresShell* rootPresShell = rootPresContext->PresShell();
-    if (nsIFrame* rootFrame = rootPresShell->GetRootFrame()) {
-      if (nsView* view = rootFrame->GetView()) {
-        LayoutDeviceToParentLayerScale parentResolution(
-          rootPresShell->GetCumulativeResolution().width
-          / rootPresShell->GetResolution().width);
-        int32_t rootAUPerDevPixel = rootPresContext->AppUnitsPerDevPixel();
-        nsRect viewBounds = view->GetBounds();
-        LayerSize viewSize = ViewAs<LayerPixel>(
-          (LayoutDeviceRect::FromAppUnits(viewBounds, rootAUPerDevPixel)
-           * parentResolution).Size(), PixelCastJustification::ParentLayerToLayerForRootComposition);
-        nsIWidget* widget =
-#ifdef MOZ_WIDGET_ANDROID
-            rootFrame->GetNearestWidget();
-#else
-            view->GetWidget();
-#endif
-        if (widget) {
-          nsIntRect widgetBounds;
-          widget->GetBounds(widgetBounds);
-          rootCompositionSize = LayerSize(ViewAs<LayerPixel>(widgetBounds.Size()));
-#ifdef MOZ_WIDGET_ANDROID
-          if (viewSize.height < rootCompositionSize.height) {
-            rootCompositionSize.height = viewSize.height;
-          }
-#endif
-        } else {
-          rootCompositionSize = viewSize;
-        }
-      }
-    }
-  } else {
-    nsIWidget* widget = (aScrollFrame ? aScrollFrame : aForFrame)->GetNearestWidget();
-    nsIntRect bounds;
-    widget->GetBounds(bounds);
-    rootCompositionSize = LayerSize(ViewAs<LayerPixel>(bounds.Size()));
-  }
-
-  // Adjust composition size for the size of scroll bars.
-  nsIFrame* rootRootScrollFrame = rootPresShell ? rootPresShell->GetRootScrollFrame() : nullptr;
-  nsIScrollableFrame* rootScrollableFrame = nullptr;
-  if (rootRootScrollFrame) {
-    rootScrollableFrame = aScrollFrame->GetScrollTargetFrame();
-  }
-  if (rootScrollableFrame && !LookAndFeel::GetInt(LookAndFeel::eIntID_UseOverlayScrollbars)) {
-    CSSMargin margins = CSSMargin::FromAppUnits(rootScrollableFrame->GetActualScrollbarSizes());
-    // Scrollbars are not subject to scaling, so CSS pixels = layer pixels for them.
-    rootCompositionSize.width -= margins.LeftRight();
-    rootCompositionSize.height -= margins.TopBottom();
-  }
-
-  return rootCompositionSize / aMetrics.LayersPixelsPerCSSPixel();
-}
-
 static void RecordFrameMetrics(nsIFrame* aForFrame,
                                nsIFrame* aScrollFrame,
                                const nsIFrame* aReferenceFrame,
@@ -858,8 +781,8 @@ static void RecordFrameMetrics(nsIFrame* aForFrame,
   }
 
   metrics.SetRootCompositionSize(
-    CalculateRootCompositionSize(metrics, isRootContentDocRootScrollFrame,
-                                 presContext, aForFrame, aScrollFrame));
+    nsLayoutUtils::CalculateRootCompositionSize(aScrollFrame ? aScrollFrame : aForFrame,
+                                                isRootContentDocRootScrollFrame, metrics));
 
   if (GetApzcTreePrintPref()) {
     if (nsIContent* content = frameForCompositionBoundsCalculation->GetContent()) {
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index a7f9cc48cd5..6cb60f0e852 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -74,6 +74,7 @@
 #include "mozilla/gfx/2D.h"
 #include "gfx2DGlue.h"
 #include "mozilla/LookAndFeel.h"
+#include "UnitTransforms.h"
 
 #include "mozilla/Preferences.h"
 
@@ -6216,6 +6217,82 @@ nsLayoutUtils::CalculateCompositionSizeForFrame(nsIFrame* aFrame)
 
   return size;
 }
+/* static */ CSSSize
+nsLayoutUtils::CalculateRootCompositionSize(nsIFrame* aFrame,
+                                            bool aIsRootContentDocRootScrollFrame,
+                                            const FrameMetrics& aMetrics)
+{
+
+  if (aIsRootContentDocRootScrollFrame) {
+    return ViewAs<LayerPixel>(ParentLayerSize(aMetrics.mCompositionBounds.Size()),
+                              PixelCastJustification::ParentLayerToLayerForRootComposition)
+           / aMetrics.LayersPixelsPerCSSPixel();
+  }
+  nsPresContext* presContext = aFrame->PresContext();
+  LayerSize rootCompositionSize;
+  nsPresContext* rootPresContext =
+    presContext->GetToplevelContentDocumentPresContext();
+  if (!rootPresContext) {
+    rootPresContext = presContext->GetRootPresContext();
+  }
+  nsIPresShell* rootPresShell = nullptr;
+  if (rootPresContext) {
+    // See the comments in the code that calculates the root
+    // composition bounds in RecordFrameMetrics.
+    // TODO: Reuse that code here.
+    nsIPresShell* rootPresShell = rootPresContext->PresShell();
+    if (nsIFrame* rootFrame = rootPresShell->GetRootFrame()) {
+      if (nsView* view = rootFrame->GetView()) {
+        LayoutDeviceToParentLayerScale parentResolution(
+          rootPresShell->GetCumulativeResolution().width
+          / rootPresShell->GetResolution().width);
+        int32_t rootAUPerDevPixel = rootPresContext->AppUnitsPerDevPixel();
+        nsRect viewBounds = view->GetBounds();
+        LayerSize viewSize = ViewAs<LayerPixel>(
+          (LayoutDeviceRect::FromAppUnits(viewBounds, rootAUPerDevPixel)
+           * parentResolution).Size(), PixelCastJustification::ParentLayerToLayerForRootComposition);
+        nsIWidget* widget =
+#ifdef MOZ_WIDGET_ANDROID
+            rootFrame->GetNearestWidget();
+#else
+            view->GetWidget();
+#endif
+        if (widget) {
+          nsIntRect widgetBounds;
+          widget->GetBounds(widgetBounds);
+          rootCompositionSize = LayerSize(ViewAs<LayerPixel>(widgetBounds.Size()));
+#ifdef MOZ_WIDGET_ANDROID
+          if (viewSize.height < rootCompositionSize.height) {
+            rootCompositionSize.height = viewSize.height;
+          }
+#endif
+        } else {
+          rootCompositionSize = viewSize;
+        }
+      }
+    }
+  } else {
+    nsIWidget* widget = aFrame->GetNearestWidget();
+    nsIntRect widgetBounds;
+    widget->GetBounds(widgetBounds);
+    rootCompositionSize = LayerSize(ViewAs<LayerPixel>(widgetBounds.Size()));
+  }
+
+  // Adjust composition size for the size of scroll bars.
+  nsIFrame* rootRootScrollFrame = rootPresShell ? rootPresShell->GetRootScrollFrame() : nullptr;
+  nsIScrollableFrame* rootScrollableFrame = nullptr;
+  if (rootRootScrollFrame) {
+    rootScrollableFrame = rootRootScrollFrame->GetScrollTargetFrame();
+  }
+  if (rootScrollableFrame && !LookAndFeel::GetInt(LookAndFeel::eIntID_UseOverlayScrollbars)) {
+    CSSMargin margins = CSSMargin::FromAppUnits(rootScrollableFrame->GetActualScrollbarSizes());
+    // Scrollbars are not subject to scaling, so CSS pixels = layer pixels for them.
+    rootCompositionSize.width -= margins.LeftRight();
+    rootCompositionSize.height -= margins.TopBottom();
+  }
+
+  return rootCompositionSize / aMetrics.LayersPixelsPerCSSPixel();
+}
 
 /* static */ nsRect
 nsLayoutUtils::CalculateScrollableRectForFrame(nsIScrollableFrame* aScrollableFrame, nsIFrame* aRootFrame)
diff --git a/layout/base/nsLayoutUtils.h b/layout/base/nsLayoutUtils.h
index f48c42ef659..fbe139ad49d 100644
--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -126,6 +126,7 @@ public:
   typedef mozilla::layers::FrameMetrics FrameMetrics;
   typedef FrameMetrics::ViewID ViewID;
   typedef mozilla::CSSPoint CSSPoint;
+  typedef mozilla::CSSSize CSSSize;
 
   /**
    * Finds previously assigned ViewID for the given content element, if any.
@@ -2100,6 +2101,22 @@ public:
   static nsSize
   CalculateCompositionSizeForFrame(nsIFrame* aFrame);
 
+ /**
+  * Calculate the composition size for the root scroll frame of the root
+  * content document.
+  * @param aFrame A frame in the root content document (or a descendant of it).
+  * @param aIsRootContentDocRootScrollFrame Whether aFrame is already the root
+  *          scroll frame of the root content document. In this case we just
+  *          use aFrame's own composition size.
+  * @param aMetrics A partially populated FrameMetrics for aFrame. Must have at
+  *          least mCompositionBounds, mCumulativeResolution, and
+  *          mDevPixelsPerCSSPixel set.
+  */
+  static CSSSize
+  CalculateRootCompositionSize(nsIFrame* aFrame,
+                               bool aIsRootContentDocRootScrollFrame,
+                               const FrameMetrics& aMetrics);
+
  /**
   * Calculate the scrollable rect for a frame. See FrameMetrics.h for
   * defintion of scrollable rect. aScrollableFrame is the scroll frame to calculate