--- a/layout/generic/nsFloatManager.cpp
+++ b/layout/generic/nsFloatManager.cpp
@@ -666,53 +666,38 @@ nsFloatManager::FloatInfo::FloatInfo(nsI
   if (shapeOutside.GetType() == StyleShapeSourceType::URL) {
     // Bug 1265343: Implement 'shape-image-threshold'. Early return
     // here because shape-outside with url() value doesn't have a
     // reference box, and GetReferenceBox() asserts that.
     return;
   }
 
   // Initialize <shape-box>'s reference rect.
-  LogicalRect rect = aMarginRect;
-
-  switch (shapeOutside.GetReferenceBox()) {
-    case StyleShapeOutsideShapeBox::Content:
-      rect.Deflate(aWM, mFrame->GetLogicalUsedPadding(aWM));
-      MOZ_FALLTHROUGH;
-    case StyleShapeOutsideShapeBox::Padding:
-      rect.Deflate(aWM, mFrame->GetLogicalUsedBorder(aWM));
-      MOZ_FALLTHROUGH;
-    case StyleShapeOutsideShapeBox::Border:
-      rect.Deflate(aWM, mFrame->GetLogicalUsedMargin(aWM));
-      break;
-    case StyleShapeOutsideShapeBox::Margin:
-      // Do nothing. rect is already a margin rect.
-      break;
-    case StyleShapeOutsideShapeBox::NoBox:
-      MOZ_ASSERT(shapeOutside.GetType() != StyleShapeSourceType::Box,
-                 "Box source type must have <shape-box> specified!");
-      break;
-  }
+  LogicalRect shapeBoxRect =
+    ShapeInfo::ComputeShapeBoxRect(shapeOutside, mFrame, aMarginRect, aWM);
 
   if (shapeOutside.GetType() == StyleShapeSourceType::Box) {
-    nsRect shapeBoxRect(rect.LineLeft(aWM, aContainerSize), rect.BStart(aWM),
-                        rect.ISize(aWM), rect.BSize(aWM));
-    mShapeInfo = MakeUnique<BoxShapeInfo>(shapeBoxRect, mFrame);
+    nsRect rect(shapeBoxRect.LineLeft(aWM, aContainerSize),
+                shapeBoxRect.BStart(aWM),
+                shapeBoxRect.ISize(aWM),
+                shapeBoxRect.BSize(aWM));
+    mShapeInfo = MakeUnique<BoxShapeInfo>(rect, mFrame);
   } else if (shapeOutside.GetType() == StyleShapeSourceType::Shape) {
     StyleBasicShape* const basicShape = shapeOutside.GetBasicShape();
 
     switch (basicShape->GetShapeType()) {
       case StyleBasicShapeType::Polygon:
         // Bug 1326409 - Implement the rendering of basic shape polygon()
         // for CSS shape-outside.
         return;
       case StyleBasicShapeType::Circle:
       case StyleBasicShapeType::Ellipse:
         mShapeInfo =
-          ShapeInfo::CreateCircleOrEllipse(basicShape, rect, aWM, aContainerSize);
+          ShapeInfo::CreateCircleOrEllipse(basicShape, shapeBoxRect, aWM,
+                                           aContainerSize);
         break;
       case StyleBasicShapeType::Inset:
         // Bug 1326407 - Implement the rendering of basic shape inset() for
         // CSS shape-outside.
         return;
     }
   } else {
     MOZ_ASSERT_UNREACHABLE("Unknown StyleShapeSourceType!");
@@ -823,16 +808,47 @@ nsFloatManager::FloatInfo::IsEmpty(Shape
     return IsEmpty();
   }
   return mShapeInfo->IsEmpty();
 }
 
 /////////////////////////////////////////////////////////////////////////////
 // ShapeInfo
 
+/* static */ LogicalRect
+nsFloatManager::ShapeInfo::ComputeShapeBoxRect(
+  const StyleShapeOutside& aShapeOutside,
+  nsIFrame* const aFrame,
+  const mozilla::LogicalRect& aMarginRect,
+  mozilla::WritingMode aWM)
+{
+  LogicalRect rect = aMarginRect;
+
+  switch (aShapeOutside.GetReferenceBox()) {
+    case StyleShapeOutsideShapeBox::Content:
+      rect.Deflate(aWM, aFrame->GetLogicalUsedPadding(aWM));
+      MOZ_FALLTHROUGH;
+    case StyleShapeOutsideShapeBox::Padding:
+      rect.Deflate(aWM, aFrame->GetLogicalUsedBorder(aWM));
+      MOZ_FALLTHROUGH;
+    case StyleShapeOutsideShapeBox::Border:
+      rect.Deflate(aWM, aFrame->GetLogicalUsedMargin(aWM));
+      break;
+    case StyleShapeOutsideShapeBox::Margin:
+      // Do nothing. rect is already a margin rect.
+      break;
+    case StyleShapeOutsideShapeBox::NoBox:
+      MOZ_ASSERT(aShapeOutside.GetType() != StyleShapeSourceType::Box,
+                 "Box source type must have <shape-box> specified!");
+      break;
+  }
+
+  return rect;
+}
+
 /* static */ UniquePtr<nsFloatManager::ShapeInfo>
 nsFloatManager::ShapeInfo::CreateCircleOrEllipse(
   StyleBasicShape* const aBasicShape,
   const LogicalRect& aShapeBoxRect,
   WritingMode aWM,
   const nsSize& aContainerSize)
 {
   // Use physical coordinates to compute the center of circle() or ellipse()

--- a/layout/generic/nsFloatManager.h
+++ b/layout/generic/nsFloatManager.h
@@ -356,16 +356,22 @@ private:
                               const nscoord aBEnd) const = 0;
     virtual nscoord BStart() const = 0;
     virtual nscoord BEnd() const = 0;
     virtual bool IsEmpty() const = 0;
 
     // Translate the current origin by the specified offsets.
     virtual void Translate(nscoord aLineLeft, nscoord aBlockStart) = 0;
 
+    static mozilla::LogicalRect ComputeShapeBoxRect(
+      const mozilla::StyleShapeOutside& aShapeOutside,
+      nsIFrame* const aFrame,
+      const mozilla::LogicalRect& aMarginRect,
+      mozilla::WritingMode aWM);
+
     static mozilla::UniquePtr<ShapeInfo> CreateCircleOrEllipse(
       mozilla::StyleBasicShape* const aBasicShape,
       const mozilla::LogicalRect& aShapeBoxRect,
       mozilla::WritingMode aWM,
       const nsSize& aContainerSize);
 
   protected:
     // Compute the minimum line-axis difference between the bounding shape