diff --git a/dom/base/ResizeObserver.cpp b/dom/base/ResizeObserver.cpp
index 0b7bf9afe5cd..a1fb0ce6daf1 100644
--- a/dom/base/ResizeObserver.cpp
+++ b/dom/base/ResizeObserver.cpp
@@ -11,7 +11,6 @@
 #include "mozilla/SVGUtils.h"
 #include "nsIContent.h"
 #include "nsIContentInlines.h"
-#include "nsIScrollableFrame.h"
 #include <limits>
 
 namespace mozilla::dom {
@@ -46,14 +45,6 @@ static uint32_t GetNodeDepth(nsINode* aNode) {
   return depth;
 }
 
-static nsSize GetContentRectSize(const nsIFrame& aFrame) {
-  if (const nsIScrollableFrame* f = do_QueryFrame(&aFrame)) {
-    // We return the scrollport rect for compat with other UAs, see bug 1733042.
-    return f->GetScrollPortRect().Size();
-  }
-  return aFrame.GetContentRectRelativeToSelf().Size();
-}
-
 /**
  * Returns |aTarget|'s size in the form of gfx::Size (in pixels).
  * If the target is SVG, width and height are determined from bounding box.
@@ -84,42 +75,47 @@ static gfx::Size CalculateBoxSize(Element* aTarget,
       const LayoutDeviceIntSize snappedSize =
           RoundedToInt(CSSSize::FromUnknownSize(size) *
                        frame->PresContext()->CSSToDevPixelScale());
-      return gfx::Size(snappedSize.ToUnknownSize());
+      size = gfx::Size(snappedSize.ToUnknownSize());
+    }
+  } else {
+    // Per the spec, non-replaced inline Elements will always have an empty
+    // content rect. Therefore, we always use the same trivially-empty size
+    // for non-replaced inline elements here, and their IsActive() will
+    // always return false. (So its observation won't be fired.)
+    if (!frame->IsFrameOfType(nsIFrame::eReplaced) &&
+        frame->IsFrameOfType(nsIFrame::eLineParticipant)) {
+      return size;
+    }
+
+    switch (aBox) {
+      case ResizeObserverBoxOptions::Border_box:
+        // GetSize() includes the content area, borders, and padding.
+        size = CSSPixel::FromAppUnits(frame->GetSize()).ToUnknownSize();
+        break;
+      case ResizeObserverBoxOptions::Device_pixel_content_box: {
+        // This is a implementation-dependent for subpixel snapping algorithm.
+        // Gecko relys on LayoutDevicePixel to convert (and snap) the app units
+        // into device pixels in painting and gfx code, so here we simply
+        // convert it into dev pixels and round it.
+        //
+        // Note: This size must contain integer values.
+        // https://drafts.csswg.org/resize-observer/#dom-resizeobserverboxoptions-device-pixel-content-box
+        const LayoutDeviceIntSize snappedSize =
+            LayoutDevicePixel::FromAppUnitsRounded(
+                frame->GetContentRectRelativeToSelf().Size(),
+                frame->PresContext()->AppUnitsPerDevPixel());
+        size = gfx::Size(snappedSize.ToUnknownSize());
+        break;
+      }
+      case ResizeObserverBoxOptions::Content_box:
+      default:
+        size =
+            CSSPixel::FromAppUnits(frame->GetContentRectRelativeToSelf().Size())
+                .ToUnknownSize();
     }
-    return size;
   }
 
-  // Per the spec, non-replaced inline Elements will always have an empty
-  // content rect. Therefore, we always use the same trivially-empty size
-  // for non-replaced inline elements here, and their IsActive() will
-  // always return false. (So its observation won't be fired.)
-  if (!frame->IsFrameOfType(nsIFrame::eReplaced) &&
-      frame->IsFrameOfType(nsIFrame::eLineParticipant)) {
-    return size;
-  }
-
-  switch (aBox) {
-    case ResizeObserverBoxOptions::Border_box:
-      return CSSPixel::FromAppUnits(frame->GetSize()).ToUnknownSize();
-    case ResizeObserverBoxOptions::Device_pixel_content_box: {
-      // This is a implementation-dependent for subpixel snapping algorithm.
-      // Gecko relies on LayoutDevicePixel to convert (and snap) the app units
-      // into device pixels in painting and gfx code, so here we simply
-      // convert it into dev pixels and round it.
-      //
-      // Note: This size must contain integer values.
-      // https://drafts.csswg.org/resize-observer/#dom-resizeobserverboxoptions-device-pixel-content-box
-      const LayoutDeviceIntSize snappedSize =
-          LayoutDevicePixel::FromAppUnitsRounded(
-              GetContentRectSize(*frame),
-              frame->PresContext()->AppUnitsPerDevPixel());
-      return gfx::Size(snappedSize.ToUnknownSize());
-    }
-    case ResizeObserverBoxOptions::Content_box:
-    default:
-      break;
-  }
-  return CSSPixel::FromAppUnits(GetContentRectSize(*frame)).ToUnknownSize();
+  return size;
 }
 
 NS_IMPL_CYCLE_COLLECTION_CLASS(ResizeObservation)
diff --git a/testing/web-platform/tests/resize-observer/scrollbars.html b/testing/web-platform/tests/resize-observer/scrollbars.html
deleted file mode 100644
index 129e74e5cd96..000000000000
--- a/testing/web-platform/tests/resize-observer/scrollbars.html
+++ /dev/null
@@ -1,56 +0,0 @@
-<!DOCTYPE html>
-<title>ResizeObserver content-box size and scrollbars</title>
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
-<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1733042">
-<style>
-  #outer {
-    position: relative;
-    width: 100px;
-    height: 200px;
-    overflow: auto;
-    background: #818182;
-  }
-
-  #inner {
-    position: absolute;
-    top: 0;
-    left: 0;
-    width: 10px;
-    height: 10px;
-    background: #0a6fc0;
-  }
-</style>
-<div id="outer">
-  <div id="inner"></div>
-</div>
-<script>
-  async function animationFrame() {
-    return new Promise(r => requestAnimationFrame(r));
-  }
-
-  // This test is expected to fail with overlay scrollbars.
-  promise_test(async function() {
-    let count = 0;
-
-    const outer = document.getElementById('outer');
-    const inner = document.getElementById('inner');
-    const observer = new ResizeObserver(entries => {
-      count++;
-    });
-
-    observer.observe(outer);
-
-    inner.style.top = '1000px';
-
-    await animationFrame();
-    await animationFrame();
-
-    inner.style.top = 0;
-
-    await animationFrame();
-    await animationFrame();
-
-    assert_equals(count, 2, "ResizeObserver should subtract scrollbar sizes from content-box rect");
-  });
-</script>