diff --git a/dom/base/ResizeObserver.cpp b/dom/base/ResizeObserver.cpp
index 856d842b469c..5ff4b85c8a09 100644
--- a/dom/base/ResizeObserver.cpp
+++ b/dom/base/ResizeObserver.cpp
@@ -74,7 +74,8 @@ static nsSize GetContentRectSize(const nsIFrame& aFrame) {
  * https://www.w3.org/TR/resize-observer-1/#calculate-box-size
  */
 static AutoTArray<LogicalPixelSize, 1> CalculateBoxSize(
-    Element* aTarget, ResizeObserverBoxOptions aBox) {
+    Element* aTarget, ResizeObserverBoxOptions aBox,
+    const ResizeObserver& aObserver) {
   nsIFrame* frame = aTarget->GetPrimaryFrame();
 
   if (!frame) {
@@ -157,7 +158,8 @@ static AutoTArray<LogicalPixelSize, 1> CalculateBoxSize(
     }
     return CSSPixel::FromAppUnits(GetContentRectSize(*aFrame)).ToUnknownSize();
   };
-  if (!StaticPrefs::dom_resize_observer_support_fragments()) {
+  if (!StaticPrefs::dom_resize_observer_support_fragments() &&
+      !aObserver.HasNativeCallback()) {
     return {LogicalPixelSize(frame->GetWritingMode(), GetFrameSize(frame))};
   }
   AutoTArray<LogicalPixelSize, 1> size;
@@ -213,7 +215,8 @@ bool ResizeObservation::IsActive() const {
     return false;
   }
 
-  return mLastReportedSize != CalculateBoxSize(mTarget, mObservedBox);
+  return mLastReportedSize !=
+         CalculateBoxSize(mTarget, mObservedBox, *mObserver);
 }
 
 void ResizeObservation::UpdateLastReportedSize(
@@ -397,11 +400,11 @@ uint32_t ResizeObserver::BroadcastActiveObservations() {
     Element* target = observation->Target();
 
     auto borderBoxSize =
-        CalculateBoxSize(target, ResizeObserverBoxOptions::Border_box);
+        CalculateBoxSize(target, ResizeObserverBoxOptions::Border_box, *this);
     auto contentBoxSize =
-        CalculateBoxSize(target, ResizeObserverBoxOptions::Content_box);
+        CalculateBoxSize(target, ResizeObserverBoxOptions::Content_box, *this);
     auto devicePixelContentBoxSize = CalculateBoxSize(
-        target, ResizeObserverBoxOptions::Device_pixel_content_box);
+        target, ResizeObserverBoxOptions::Device_pixel_content_box, *this);
     RefPtr<ResizeObserverEntry> entry =
         new ResizeObserverEntry(mOwner, *target, borderBoxSize, contentBoxSize,
                                 devicePixelContentBoxSize);
diff --git a/dom/base/ResizeObserver.h b/dom/base/ResizeObserver.h
index 9f8ec87dd3e5..6f1fc5b6cda3 100644
--- a/dom/base/ResizeObserver.h
+++ b/dom/base/ResizeObserver.h
@@ -179,7 +179,7 @@ class ResizeObserver final : public nsISupports, public nsWrapperCache {
   /**
    * Returns whether this is an internal ResizeObserver with a native callback.
    */
-  bool HasNativeCallback() { return mCallback.is<NativeCallback>(); }
+  bool HasNativeCallback() const { return mCallback.is<NativeCallback>(); }
 
   /**
    * Invoke the callback function in JavaScript for all active observations
diff --git a/testing/web-platform/meta/css/css-sizing/contain-intrinsic-size/__dir__.ini b/testing/web-platform/meta/css/css-sizing/contain-intrinsic-size/__dir__.ini
index ad6d6e1d8089..e977af25bca7 100644
--- a/testing/web-platform/meta/css/css-sizing/contain-intrinsic-size/__dir__.ini
+++ b/testing/web-platform/meta/css/css-sizing/contain-intrinsic-size/__dir__.ini
@@ -1 +1 @@
-prefs: [layout.css.content-visibility.enabled:true, dom.resize_observer.support_fragments:true]
+prefs: [layout.css.content-visibility.enabled:true]