diff --git a/dom/base/nsDOMWindowUtils.cpp b/dom/base/nsDOMWindowUtils.cpp
index 059ccd87b163..1cb573a0d631 100644
--- a/dom/base/nsDOMWindowUtils.cpp
+++ b/dom/base/nsDOMWindowUtils.cpp
@@ -558,8 +558,10 @@ nsDOMWindowUtils::GetScrollbarSizes(Element* aElement,
     return NS_ERROR_INVALID_ARG;
   }
 
-  CSSIntMargin scrollbarSizes = RoundedToInt(
-      CSSMargin::FromAppUnits(scrollFrame->GetActualScrollbarSizes()));
+  CSSIntMargin scrollbarSizes =
+      RoundedToInt(CSSMargin::FromAppUnits(scrollFrame->GetActualScrollbarSizes(
+          nsIScrollableFrame::ScrollbarSizesOptions::
+              INCLUDE_VISUAL_VIEWPORT_SCROLLBARS)));
   *aOutVerticalScrollbarWidth = scrollbarSizes.LeftRight();
   *aOutHorizontalScrollbarHeight = scrollbarSizes.TopBottom();
 
diff --git a/dom/interfaces/base/nsIDOMWindowUtils.idl b/dom/interfaces/base/nsIDOMWindowUtils.idl
index 2e3a54bf94f3..6b884d42bb69 100644
--- a/dom/interfaces/base/nsIDOMWindowUtils.idl
+++ b/dom/interfaces/base/nsIDOMWindowUtils.idl
@@ -205,6 +205,11 @@ interface nsIDOMWindowUtils : nsISupports {
    * Note that on some platforms, scrollbars don't take up layout space
    * ("overlay scrollbars"). On such platforms, the returned sizes are
    * always zero.
+   *
+   * Layout scrollbars that normally take up space but were only shown to
+   * scroll the visual viewport inside the layout viewport (the layout viewport
+   * cannot be scrolled) do not take up space but they still return their size
+   * from this function.
    */
   void getScrollbarSizes(in Element aElement,
                          out uint32_t aVerticalScrollbarWidth,
diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 757aeb95277d..b39b8df5fba1 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -8798,7 +8798,9 @@ nsMargin nsLayoutUtils::ScrollbarAreaToExcludeFromCompositionBoundsFor(
   if (!scrollableFrame) {
     return nsMargin();
   }
-  return scrollableFrame->GetActualScrollbarSizes();
+  return scrollableFrame->GetActualScrollbarSizes(
+      nsIScrollableFrame::ScrollbarSizesOptions::
+          INCLUDE_VISUAL_VIEWPORT_SCROLLBARS);
 }
 
 /* static */
diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index ad2438bb013b..a382dadd306f 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -6905,11 +6905,38 @@ nsRect ScrollFrameHelper::GetUnsnappedScrolledRectInternal(
                                         aScrollPortSize, GetScrolledFrameDir());
 }
 
-nsMargin ScrollFrameHelper::GetActualScrollbarSizes() const {
+nsMargin ScrollFrameHelper::GetActualScrollbarSizes(
+    nsIScrollableFrame::ScrollbarSizesOptions
+        aOptions /* = nsIScrollableFrame::ScrollbarSizesOptions::NONE */)
+    const {
   nsRect r = mOuter->GetPaddingRectRelativeToSelf();
 
-  return nsMargin(mScrollPort.y - r.y, r.XMost() - mScrollPort.XMost(),
-                  r.YMost() - mScrollPort.YMost(), mScrollPort.x - r.x);
+  nsMargin m(mScrollPort.y - r.y, r.XMost() - mScrollPort.XMost(),
+             r.YMost() - mScrollPort.YMost(), mScrollPort.x - r.x);
+
+  if (aOptions == nsIScrollableFrame::ScrollbarSizesOptions::
+                      INCLUDE_VISUAL_VIEWPORT_SCROLLBARS &&
+      !UsesOverlayScrollbars()) {
+    // If we are using layout scrollbars and they only exist to scroll the
+    // visual viewport then they do not take up any layout space (so the
+    // scrollport is the same as the padding rect) but they do cover everything
+    // below them so some callers may want to include this special type of
+    // scrollbars in the returned value.
+    if (mHScrollbarBox && mHasHorizontalScrollbar &&
+        mOnlyNeedHScrollbarToScrollVVInsideLV) {
+      m.bottom += mHScrollbarBox->GetRect().height;
+    }
+    if (mVScrollbarBox && mHasVerticalScrollbar &&
+        mOnlyNeedVScrollbarToScrollVVInsideLV) {
+      if (IsScrollbarOnRight()) {
+        m.right += mVScrollbarBox->GetRect().width;
+      } else {
+        m.left += mVScrollbarBox->GetRect().width;
+      }
+    }
+  }
+
+  return m;
 }
 
 void ScrollFrameHelper::SetScrollbarVisibility(nsIFrame* aScrollbar,
diff --git a/layout/generic/nsGfxScrollFrame.h b/layout/generic/nsGfxScrollFrame.h
index c3bb32ccdb31..285c4fd085be 100644
--- a/layout/generic/nsGfxScrollFrame.h
+++ b/layout/generic/nsGfxScrollFrame.h
@@ -344,7 +344,9 @@ class ScrollFrameHelper : public nsIReflowCallback {
     return (mHasVerticalScrollbar ? nsIScrollableFrame::VERTICAL : 0) |
            (mHasHorizontalScrollbar ? nsIScrollableFrame::HORIZONTAL : 0);
   }
-  nsMargin GetActualScrollbarSizes() const;
+  nsMargin GetActualScrollbarSizes(
+      nsIScrollableFrame::ScrollbarSizesOptions aOptions =
+          nsIScrollableFrame::ScrollbarSizesOptions::NONE) const;
   nsMargin GetDesiredScrollbarSizes(nsBoxLayoutState* aState);
   nscoord GetNondisappearingScrollbarWidth(nsBoxLayoutState* aState,
                                            mozilla::WritingMode aVerticalWM);
@@ -905,8 +907,10 @@ class nsHTMLScrollFrame : public nsContainerFrame,
   uint32_t GetScrollbarVisibility() const final {
     return mHelper.GetScrollbarVisibility();
   }
-  nsMargin GetActualScrollbarSizes() const final {
-    return mHelper.GetActualScrollbarSizes();
+  nsMargin GetActualScrollbarSizes(
+      nsIScrollableFrame::ScrollbarSizesOptions aOptions =
+          nsIScrollableFrame::ScrollbarSizesOptions::NONE) const final {
+    return mHelper.GetActualScrollbarSizes(aOptions);
   }
   nsMargin GetDesiredScrollbarSizes(nsBoxLayoutState* aState) final {
     return mHelper.GetDesiredScrollbarSizes(aState);
@@ -1385,8 +1389,10 @@ class nsXULScrollFrame final : public nsBoxFrame,
   uint32_t GetScrollbarVisibility() const final {
     return mHelper.GetScrollbarVisibility();
   }
-  nsMargin GetActualScrollbarSizes() const final {
-    return mHelper.GetActualScrollbarSizes();
+  nsMargin GetActualScrollbarSizes(
+      nsIScrollableFrame::ScrollbarSizesOptions aOptions =
+          nsIScrollableFrame::ScrollbarSizesOptions::NONE) const final {
+    return mHelper.GetActualScrollbarSizes(aOptions);
   }
   nsMargin GetDesiredScrollbarSizes(nsBoxLayoutState* aState) final {
     return mHelper.GetDesiredScrollbarSizes(aState);
diff --git a/layout/generic/nsIScrollableFrame.h b/layout/generic/nsIScrollableFrame.h
index 59c0dea40a0a..48f7abbdab2d 100644
--- a/layout/generic/nsIScrollableFrame.h
+++ b/layout/generic/nsIScrollableFrame.h
@@ -114,8 +114,14 @@ class nsIScrollableFrame : public nsIScrollbarMediator {
    * positions that don't have a scrollbar or where the scrollbar is not
    * visible. Do not call this while this frame's descendants are being
    * reflowed, it won't be accurate.
+   * INCLUDE_VISUAL_VIEWPORT_SCROLLBARS means we include the size of layout
+   * scrollbars that are only visible to scroll the visual viewport inside the
+   * layout viewport (ie the layout viewport cannot be scrolled) even though
+   * there is no layout space set aside for these scrollbars.
    */
-  virtual nsMargin GetActualScrollbarSizes() const = 0;
+  enum class ScrollbarSizesOptions { NONE, INCLUDE_VISUAL_VIEWPORT_SCROLLBARS };
+  virtual nsMargin GetActualScrollbarSizes(
+      ScrollbarSizesOptions aOptions = ScrollbarSizesOptions::NONE) const = 0;
   /**
    * Return the sizes of all scrollbars assuming that any scrollbars that could
    * be visible due to overflowing content, are. This can be called during