--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -4101,21 +4101,21 @@ nsLayoutUtils::GetFirstNonAnonymousFrame
     }
 
     aFrame = nsLayoutUtils::GetNextContinuationOrIBSplitSibling(aFrame);
   }
   return nullptr;
 }
 
 struct BoxToRect : public nsLayoutUtils::BoxCallback {
-  nsIFrame* mRelativeTo;
+  const nsIFrame* mRelativeTo;
   nsLayoutUtils::RectCallback* mCallback;
   uint32_t mFlags;
 
-  BoxToRect(nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback,
+  BoxToRect(const nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback,
             uint32_t aFlags)
     : mRelativeTo(aRelativeTo), mCallback(aCallback), mFlags(aFlags) {}
 
   virtual void AddBox(nsIFrame* aFrame) override {
     nsRect r;
     nsIFrame* outer = nsSVGUtils::GetOuterSVGFrameAndCoveredRegion(aFrame, &r);
     if (!outer) {
       outer = aFrame;
@@ -4140,17 +4140,17 @@ struct BoxToRect : public nsLayoutUtils:
     }
     mCallback->AddRect(r);
   }
 };
 
 struct MOZ_RAII BoxToRectAndText : public BoxToRect {
   Sequence<nsString>* mTextList;
 
-  BoxToRectAndText(nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback,
+  BoxToRectAndText(const nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback,
                    Sequence<nsString>* aTextList, uint32_t aFlags)
     : BoxToRect(aRelativeTo, aCallback, aFlags), mTextList(aTextList) {}
 
   static void AccumulateText(nsIFrame* aFrame, nsAString& aResult) {
     MOZ_ASSERT(aFrame);
 
     // Get all the text in aFrame and child frames, while respecting
     // the content offsets in each of the nsTextFrames.
@@ -4180,25 +4180,26 @@ struct MOZ_RAII BoxToRectAndText : publi
       if (textForFrame) {
         AccumulateText(aFrame, *textForFrame);
       }
     }
   }
 };
 
 void
-nsLayoutUtils::GetAllInFlowRects(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+nsLayoutUtils::GetAllInFlowRects(nsIFrame* aFrame, const nsIFrame* aRelativeTo,
                                  RectCallback* aCallback, uint32_t aFlags)
 {
   BoxToRect converter(aRelativeTo, aCallback, aFlags);
   GetAllInFlowBoxes(aFrame, &converter);
 }
 
 void
-nsLayoutUtils::GetAllInFlowRectsAndTexts(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+nsLayoutUtils::GetAllInFlowRectsAndTexts(nsIFrame* aFrame,
+                                         const nsIFrame* aRelativeTo,
                                          RectCallback* aCallback,
                                          Sequence<nsString>* aTextList,
                                          uint32_t aFlags)
 {
   BoxToRectAndText converter(aRelativeTo, aCallback, aTextList, aFlags);
   GetAllInFlowBoxes(aFrame, &converter);
 }
 
@@ -4225,17 +4226,17 @@ void nsLayoutUtils::RectListBuilder::Add
 }
 
 nsIFrame* nsLayoutUtils::GetContainingBlockForClientRect(nsIFrame* aFrame)
 {
   return aFrame->PresShell()->GetRootFrame();
 }
 
 nsRect
-nsLayoutUtils::GetAllInFlowRectsUnion(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+nsLayoutUtils::GetAllInFlowRectsUnion(nsIFrame* aFrame, const nsIFrame* aRelativeTo,
                                       uint32_t aFlags) {
   RectAccumulator accumulator;
   GetAllInFlowRects(aFrame, aRelativeTo, &accumulator, aFlags);
   return accumulator.mResultRect.IsEmpty() ? accumulator.mFirstRect
           : accumulator.mResultRect;
 }
 
 nsRect

--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -1216,35 +1216,36 @@ public:
    * For SVG frames, returns one rectangle, the bounding box.
    * If aFlags includes RECTS_ACCOUNT_FOR_TRANSFORMS, then when converting
    * the boxes into aRelativeTo coordinates, transforms (including CSS
    * and SVG transforms) are taken into account.
    * If aFlags includes one of RECTS_USE_CONTENT_BOX, RECTS_USE_PADDING_BOX,
    * or RECTS_USE_MARGIN_BOX, the corresponding type of box is used.
    * Otherwise (by default), the border box is used.
    */
-  static void GetAllInFlowRects(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+  static void GetAllInFlowRects(nsIFrame* aFrame, const nsIFrame* aRelativeTo,
                                 RectCallback* aCallback, uint32_t aFlags = 0);
 
-  static void GetAllInFlowRectsAndTexts(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+  static void GetAllInFlowRectsAndTexts(nsIFrame* aFrame,
+                                        const nsIFrame* aRelativeTo,
                                         RectCallback* aCallback,
                                         mozilla::dom::Sequence<nsString>* aTextList,
                                         uint32_t aFlags = 0);
 
   /**
    * Computes the union of all rects returned by GetAllInFlowRects. If
    * the union is empty, returns the first rect.
    * If aFlags includes RECTS_ACCOUNT_FOR_TRANSFORMS, then when converting
    * the boxes into aRelativeTo coordinates, transforms (including CSS
    * and SVG transforms) are taken into account.
    * If aFlags includes one of RECTS_USE_CONTENT_BOX, RECTS_USE_PADDING_BOX,
    * or RECTS_USE_MARGIN_BOX, the corresponding type of box is used.
    * Otherwise (by default), the border box is used.
    */
-  static nsRect GetAllInFlowRectsUnion(nsIFrame* aFrame, nsIFrame* aRelativeTo,
+  static nsRect GetAllInFlowRectsUnion(nsIFrame* aFrame, const nsIFrame* aRelativeTo,
                                        uint32_t aFlags = 0);
 
   enum {
     EXCLUDE_BLUR_SHADOWS = 0x01
   };
   /**
    * Takes a text-shadow array from the style properties of a given nsIFrame and
    * computes the union of those shadows along with the given initial rect.