diff --git a/layout/generic/nsGfxScrollFrame.cpp b/layout/generic/nsGfxScrollFrame.cpp
index f03c7595e82d..3812a49dc0f5 100644
--- a/layout/generic/nsGfxScrollFrame.cpp
+++ b/layout/generic/nsGfxScrollFrame.cpp
@@ -3706,7 +3706,9 @@ GetScrollPortSizeExcludingHeadersAndFooters(nsIFrame* aViewportFrame,
   for (nsFrameList::Enumerator iterator(fixedFrames); !iterator.AtEnd();
        iterator.Next()) {
     nsIFrame* f = iterator.get();
-    nsRect r = f->GetRect().Intersect(aScrollPort);
+    nsRect r = f->GetRectRelativeToSelf();
+    r = nsLayoutUtils::TransformFrameRectToAncestor(f, r, aViewportFrame);
+    r = r.Intersect(aScrollPort);
     if ((r.width >= aScrollPort.width / 2 ||
          r.width >= NSIntPixelsToAppUnits(800, AppUnitsPerCSSPixel())) &&
         r.height <= aScrollPort.height/3) {
diff --git a/layout/generic/test/page_scroll_with_fixed_pos_window.html b/layout/generic/test/page_scroll_with_fixed_pos_window.html
index 91347f9c8509..8647e7252dea 100644
--- a/layout/generic/test/page_scroll_with_fixed_pos_window.html
+++ b/layout/generic/test/page_scroll_with_fixed_pos_window.html
@@ -91,10 +91,19 @@ function runTest() {
                  "Don't ignore elements that span more than half the viewport side");
               document.getElementById("top").style.width = "100%";
 
-              // Scroll back up so test results are visible
-              document.documentElement.scrollTop = 0;
-              SimpleTest.finish();
-              window.close();
+              document.getElementById("top").style.top = "-40px";
+              document.getElementById("top").style.transform = "translateY(38px)";
+              scrollDownOnePageWithContinuation(function() {
+                is(document.documentElement.scrollTop,
+                   fullPageScrollDown - (10 + 13 - 40 + 38),
+                   "Account for offset and transform");
+                document.getElementById("top").style.width = "100%";
+
+                // Scroll back up so test results are visible
+                document.documentElement.scrollTop = 0;
+                SimpleTest.finish();
+                window.close();
+              });
             });
           });
         });