--- a/dom/base/DOMIntersectionObserver.cpp
+++ b/dom/base/DOMIntersectionObserver.cpp
@@ -263,27 +263,31 @@ DOMIntersectionObserver::Update(nsIDocum
   Element* root = nullptr;
   nsIFrame* rootFrame = nullptr;
   nsRect rootRect;
 
   if (mRoot) {
     root = mRoot;
     rootFrame = root->GetPrimaryFrame();
     if (rootFrame) {
+      nsRect rootRectRelativeToRootFrame;
       if (rootFrame->IsScrollFrame()) {
+        // rootRectRelativeToRootFrame should be the content rect of rootFrame, not including the scrollbars.
         nsIScrollableFrame* scrollFrame = do_QueryFrame(rootFrame);
-        rootRect = nsLayoutUtils::TransformFrameRectToAncestor(
-          rootFrame,
-          rootFrame->GetContentRectRelativeToSelf(),
-          scrollFrame->GetScrolledFrame());
+        rootRectRelativeToRootFrame = scrollFrame->GetScrollPortRect();
       } else {
-        rootRect = nsLayoutUtils::GetAllInFlowRectsUnion(rootFrame,
-          nsLayoutUtils::GetContainingBlockForClientRect(rootFrame),
-          nsLayoutUtils::RECTS_ACCOUNT_FOR_TRANSFORMS);
+        // rootRectRelativeToRootFrame should be the border rect of rootFrame.
+        rootRectRelativeToRootFrame = rootFrame->GetRectRelativeToSelf();
       }
+      nsIFrame* containingBlock =
+        nsLayoutUtils::GetContainingBlockForClientRect(rootFrame);
+      rootRect =
+        nsLayoutUtils::TransformFrameRectToAncestor(rootFrame,
+                                                    rootRectRelativeToRootFrame,
+                                                    containingBlock);
     }
   } else {
     nsCOMPtr<nsIPresShell> presShell = aDocument->GetShell();
     if (presShell) {
       rootFrame = presShell->GetRootScrollFrame();
       if (rootFrame) {
         nsPresContext* presContext = rootFrame->PresContext();
         while (!presContext->IsRootContentDocument()) {

--- a/testing/web-platform/meta/intersection-observer/containing-block.html.ini
+++ b/testing/web-platform/meta/intersection-observer/containing-block.html.ini
@@ -1,3 +1,2 @@
 [containing-block.html]
   type: testharness
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1363650

--- a/testing/web-platform/meta/intersection-observer/remove-element.html.ini
+++ b/testing/web-platform/meta/intersection-observer/remove-element.html.ini
@@ -1,3 +1,2 @@
 [remove-element.html]
   type: testharness
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1363650

--- a/testing/web-platform/meta/intersection-observer/same-document-root.html.ini
+++ b/testing/web-platform/meta/intersection-observer/same-document-root.html.ini
@@ -1,3 +1,2 @@
 [same-document-root.html]
   type: testharness
-  disabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1363650