diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 1c9adc36ca51b..b849e37d7ec28 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -3197,6 +3197,12 @@ nsRect nsLayoutUtils::TransformFrameRectToAncestor(
     Maybe<Matrix4x4Flagged>* aMatrixCache /* = nullptr */,
     bool aStopAtStackingContextAndDisplayPortAndOOFFrame /* = false */,
     nsIFrame** aOutAncestor /* = nullptr */) {
+#if 0
+  // FIXME(bug 1668156): This should hold.
+  MOZ_ASSERT(IsAncestorFrameCrossDoc(aAncestor.mFrame, aFrame),
+             "Fix the caller");
+#endif
+
   SVGTextFrame* text = GetContainingSVGTextFrame(aFrame);
 
   float srcAppUnitsPerDevPixel = aFrame->PresContext()->AppUnitsPerDevPixel();
@@ -4565,10 +4571,12 @@ struct BoxToRect : public nsLayoutUtils::BoxCallback {
   const nsIFrame* mRelativeTo;
   nsLayoutUtils::RectCallback* mCallback;
   uint32_t mFlags;
+  bool mRelativeToIsRoot;
 
   BoxToRect(const nsIFrame* aRelativeTo, nsLayoutUtils::RectCallback* aCallback,
             uint32_t aFlags)
-      : mRelativeTo(aRelativeTo), mCallback(aCallback), mFlags(aFlags) {}
+      : mRelativeTo(aRelativeTo), mCallback(aCallback), mFlags(aFlags),
+        mRelativeToIsRoot(!aRelativeTo->GetParent()) {}
 
   virtual void AddBox(nsIFrame* aFrame) override {
     nsRect r;
@@ -4590,7 +4598,11 @@ struct BoxToRect : public nsLayoutUtils::BoxCallback {
       }
     }
     if (mFlags & nsLayoutUtils::RECTS_ACCOUNT_FOR_TRANSFORMS) {
-      r = nsLayoutUtils::TransformFrameRectToAncestor(outer, r, mRelativeTo);
+      if (mRelativeToIsRoot) {
+        r = nsLayoutUtils::TransformFrameRectToAncestor(outer, r, mRelativeTo);
+      } else {
+        nsLayoutUtils::TransformRect(outer, mRelativeTo, r);
+      }
     } else {
       r += outer->GetOffsetTo(mRelativeTo);
     }
diff --git a/testing/web-platform/tests/intersection-observer/intersection-ratio-ib-split.html b/testing/web-platform/tests/intersection-observer/intersection-ratio-ib-split.html
index 905ea436fd6a8..1cba6daf6e8eb 100644
--- a/testing/web-platform/tests/intersection-observer/intersection-ratio-ib-split.html
+++ b/testing/web-platform/tests/intersection-observer/intersection-ratio-ib-split.html
@@ -26,6 +26,16 @@
     assert_equals(entries.length, 1, element.nodeName + ": Should get an entry");
     assert_true(entries[0].isIntersecting, element.nodeName + ": Should be intersecting");
     assert_equals(entries[0].intersectionRatio, 1, element.nodeName + ": Should be fully intersecting");
+
+    function assert_rects_equal(r1, r2, label) {
+      assert_equals(r1.top, r2.top, label + ": top should be equal");
+      assert_equals(r1.right, r2.right, label + ": right should be equal");
+      assert_equals(r1.bottom, r2.bottom, label + ": bottom should be equal");
+      assert_equals(r1.left, r2.left, label + ": left should be equal");
+    }
+
+    assert_rects_equal(entries[0].boundingClientRect, element.getBoundingClientRect(), element.nodeName + ": boundingClientRect should match");
+    assert_rects_equal(entries[0].intersectionRect, entries[0].boundingClientRect, element.nodeName + ": intersectionRect should match entry.boundingClientRect");
   }
 }, "IntersectionObserver on an IB split gets the right intersection ratio");
 </script>